diff --git a/gtsam/nonlinear/NonlinearFactor.h b/gtsam/nonlinear/NonlinearFactor.h
index e1b343218..c58179db3 100644
--- a/gtsam/nonlinear/NonlinearFactor.h
+++ b/gtsam/nonlinear/NonlinearFactor.h
@@ -555,31 +555,38 @@ class NoiseModelFactorN : public NoiseModelFactor {
   }
 
  public:
-  /// @name Deprecated methods
+  /// @name Deprecated methods.  Use `key<1>()`, `key<2>()`, ... instead of old
+  /// `key1()`, `key2()`, ...
+  /// If your class is templated AND you are trying to call `key<1>` inside your
+  /// class, due to dependent types you need to do `this->template key<1>()`.
   /// @{
 
-  template <int I = N, typename = typename std::enable_if<(I > 1), void>::type>
   inline Key GTSAM_DEPRECATED key1() const {
     return key<1>();
   }
-  template <int I = 2, typename = IndexIsValid<I>>
+  template <int I = 2>
   inline Key GTSAM_DEPRECATED key2() const {
+    static_assert(I <= N, "Index out of bounds");
     return key<2>();
   }
-  template <int I = 3, typename = IndexIsValid<I>>
+  template <int I = 3>
   inline Key GTSAM_DEPRECATED key3() const {
+    static_assert(I <= N, "Index out of bounds");
     return key<3>();
   }
-  template <int I = 4, typename = IndexIsValid<I>>
+  template <int I = 4>
   inline Key GTSAM_DEPRECATED key4() const {
+    static_assert(I <= N, "Index out of bounds");
     return key<4>();
   }
-  template <int I = 5, typename = IndexIsValid<I>>
+  template <int I = 5>
   inline Key GTSAM_DEPRECATED key5() const {
+    static_assert(I <= N, "Index out of bounds");
     return key<5>();
   }
-  template <int I = 6, typename = IndexIsValid<I>>
+  template <int I = 6>
   inline Key GTSAM_DEPRECATED key6() const {
+    static_assert(I <= N, "Index out of bounds");
     return key<6>();
   }
 
@@ -588,8 +595,12 @@ class NoiseModelFactorN : public NoiseModelFactor {
 };  // \class NoiseModelFactorN
 
 /* ************************************************************************* */
-/** @deprecated: use NoiseModelFactorN, replacing .key() with .key<1> and X1
- * with ValueType<1>.  If your class is templated, use `this->template key<1>()`
+/** @deprecated: use NoiseModelFactorN, replacing .key() with .key<1>() and X1
+ * with ValueType<1>.
+ * If your class is templated AND you are trying to call `.key<1>()` or
+ * `ValueType<1>` inside your class, due to dependent types you need to do
+ * `this->template key<1>()` or `This::template ValueType<1>`.
+ * ~~~
  * A convenient base class for creating your own NoiseModelFactor
  * with 1 variable.  To derive from this class, implement evaluateError().
  */
@@ -613,11 +624,6 @@ class GTSAM_DEPRECATED NoiseModelFactor1 : public NoiseModelFactorN<VALUE> {
   using NoiseModelFactorN<VALUE>::NoiseModelFactorN;
   ~NoiseModelFactor1() override {}
 
-  /** Method to retrieve key.
-   * Similar to `ValueType`, you can probably do `return key<1>();`
-   */
-  inline Key key() const { return NoiseModelFactorN<VALUE>::template key<1>(); }
-
  private:
   /** Serialization function */
   friend class boost::serialization::access;
@@ -629,8 +635,12 @@ class GTSAM_DEPRECATED NoiseModelFactor1 : public NoiseModelFactorN<VALUE> {
 };  // \class NoiseModelFactor1
 
 /* ************************************************************************* */
-/** @deprecated: use NoiseModelFactorN, replacing .key1() with .key<1> and X1
- * with ValueType<1>.  If your class is templated, use `this->template key<1>()`
+/** @deprecated: use NoiseModelFactorN, replacing .key1() with .key<1>() and X1
+ * with ValueType<1>.
+ * If your class is templated AND you are trying to call `.key<1>()` or
+ * `ValueType<1>` inside your class, due to dependent types you need to do
+ * `this->template key<1>()` or `This::template ValueType<1>`.
+ * ~~~
  * A convenient base class for creating your own NoiseModelFactor
  * with 2 variables.  To derive from this class, implement evaluateError().
  */
@@ -656,12 +666,6 @@ class GTSAM_DEPRECATED NoiseModelFactor2
   using NoiseModelFactorN<VALUE1, VALUE2>::NoiseModelFactorN;
   ~NoiseModelFactor2() override {}
 
-  /** Methods to retrieve keys.
-   * Similar to `ValueType`, you can probably do `return key<#>();`
-   */
-  inline Key key1() const { return this->template key<1>(); }
-  inline Key key2() const { return this->template key<2>(); }
-
  private:
   /** Serialization function */
   friend class boost::serialization::access;
@@ -673,8 +677,12 @@ class GTSAM_DEPRECATED NoiseModelFactor2
 };  // \class NoiseModelFactor2
 
 /* ************************************************************************* */
-/** @deprecated: use NoiseModelFactorN, replacing .key1() with .key<1> and X1
- * with ValueType<1>.  If your class is templated, use `this->template key<1>()`
+/** @deprecated: use NoiseModelFactorN, replacing .key1() with .key<1>() and X1
+ * with ValueType<1>.
+ * If your class is templated AND you are trying to call `.key<1>()` or
+ * `ValueType<1>` inside your class, due to dependent types you need to do
+ * `this->template key<1>()` or `This::template ValueType<1>`.
+ * ~~~
  * A convenient base class for creating your own NoiseModelFactor
  * with 3 variables.  To derive from this class, implement evaluateError().
  */
@@ -701,13 +709,6 @@ class GTSAM_DEPRECATED NoiseModelFactor3
   using NoiseModelFactorN<VALUE1, VALUE2, VALUE3>::NoiseModelFactorN;
   ~NoiseModelFactor3() override {}
 
-  /** Methods to retrieve keys.
-   * Similar to `ValueType`, you can probably do `return key<#>();`
-   */
-  inline Key key1() const { return this->template key<1>(); }
-  inline Key key2() const { return this->template key<2>(); }
-  inline Key key3() const { return this->template key<3>(); }
-
  private:
   /** Serialization function */
   friend class boost::serialization::access;
@@ -719,8 +720,12 @@ class GTSAM_DEPRECATED NoiseModelFactor3
 };  // \class NoiseModelFactor3
 
 /* ************************************************************************* */
-/** @deprecated: use NoiseModelFactorN, replacing .key1() with .key<1> and X1
- * with ValueType<1>.  If your class is templated, use `this->template key<1>()`
+/** @deprecated: use NoiseModelFactorN, replacing .key1() with .key<1>() and X1
+ * with ValueType<1>.
+ * If your class is templated AND you are trying to call `.key<1>()` or
+ * `ValueType<1>` inside your class, due to dependent types you need to do
+ * `this->template key<1>()` or `This::template ValueType<1>`.
+ * ~~~
  * A convenient base class for creating your own NoiseModelFactor
  * with 4 variables.  To derive from this class, implement evaluateError().
  */
@@ -748,14 +753,6 @@ class GTSAM_DEPRECATED NoiseModelFactor4
   using NoiseModelFactorN<VALUE1, VALUE2, VALUE3, VALUE4>::NoiseModelFactorN;
   ~NoiseModelFactor4() override {}
 
-  /** Methods to retrieve keys.
-   * Similar to `ValueType`, you can probably do `return key<#>();`
-   */
-  inline Key key1() const { return this->template key<1>(); }
-  inline Key key2() const { return this->template key<2>(); }
-  inline Key key3() const { return this->template key<3>(); }
-  inline Key key4() const { return this->template key<4>(); }
-
  private:
   /** Serialization function */
   friend class boost::serialization::access;
@@ -767,8 +764,12 @@ class GTSAM_DEPRECATED NoiseModelFactor4
 };  // \class NoiseModelFactor4
 
 /* ************************************************************************* */
-/** @deprecated: use NoiseModelFactorN, replacing .key1() with .key<1> and X1
- * with ValueType<1>.  If your class is templated, use `this->template key<1>()`
+/** @deprecated: use NoiseModelFactorN, replacing .key1() with .key<1>() and X1
+ * with ValueType<1>.
+ * If your class is templated AND you are trying to call `.key<1>()` or
+ * `ValueType<1>` inside your class, due to dependent types you need to do
+ * `this->template key<1>()` or `This::template ValueType<1>`.
+ * ~~~
  * A convenient base class for creating your own NoiseModelFactor
  * with 5 variables.  To derive from this class, implement evaluateError().
  */
@@ -798,15 +799,6 @@ class GTSAM_DEPRECATED NoiseModelFactor5
                           VALUE5>::NoiseModelFactorN;
   ~NoiseModelFactor5() override {}
 
-  /** Methods to retrieve keys.
-   * Similar to `ValueType`, you can probably do `return key<#>();`
-   */
-  inline Key key1() const { return this->template key<1>(); }
-  inline Key key2() const { return this->template key<2>(); }
-  inline Key key3() const { return this->template key<3>(); }
-  inline Key key4() const { return this->template key<4>(); }
-  inline Key key5() const { return this->template key<5>(); }
-
  private:
   /** Serialization function */
   friend class boost::serialization::access;
@@ -818,8 +810,12 @@ class GTSAM_DEPRECATED NoiseModelFactor5
 };  // \class NoiseModelFactor5
 
 /* ************************************************************************* */
-/** @deprecated: use NoiseModelFactorN, replacing .key1() with .key<1> and X1
- * with ValueType<1>.  If your class is templated, use `this->template key<1>()`
+/** @deprecated: use NoiseModelFactorN, replacing .key1() with .key<1>() and X1
+ * with ValueType<1>.
+ * If your class is templated AND you are trying to call `.key<1>()` or
+ * `ValueType<1>` inside your class, due to dependent types you need to do
+ * `this->template key<1>()` or `This::template ValueType<1>`.
+ * ~~~
  * A convenient base class for creating your own NoiseModelFactor
  * with 6 variables.  To derive from this class, implement evaluateError().
  */
@@ -852,16 +848,6 @@ class GTSAM_DEPRECATED NoiseModelFactor6
                           VALUE6>::NoiseModelFactorN;
   ~NoiseModelFactor6() override {}
 
-  /** Methods to retrieve keys.
-   * Similar to `ValueType`, you can probably do `return key<#>();`
-   */
-  inline Key key1() const { return this->template key<1>(); }
-  inline Key key2() const { return this->template key<2>(); }
-  inline Key key3() const { return this->template key<3>(); }
-  inline Key key4() const { return this->template key<4>(); }
-  inline Key key5() const { return this->template key<5>(); }
-  inline Key key6() const { return this->template key<6>(); }
-
  private:
   /** Serialization function */
   friend class boost::serialization::access;