diff --git a/config/method.go b/config/method.go
index 9db813c..f175758 100644
--- a/config/method.go
+++ b/config/method.go
@@ -87,6 +87,7 @@ func parseMethodLine(ctx *context, c *Converter, m *Method, value string) (err e
 				OutputPackagePath: c.OutputPackagePath,
 				Converter:         c.Type,
 				Params:            method.ParamsOptional,
+				AllowTypeParams:   true,
 			}
 			f.Function, err = ctx.Loader.GetOne(c.Package, custom, opts)
 		}
@@ -129,6 +130,7 @@ func parseMethodLine(ctx *context, c *Converter, m *Method, value string) (err e
 			OutputPackagePath: c.OutputPackagePath,
 			Converter:         c.Type,
 			Params:            method.ParamsOptional,
+			AllowTypeParams:   true,
 		}
 		m.Constructor, err = ctx.Loader.GetOne(c.Package, rest, opts)
 	default:
diff --git a/generator/generator.go b/generator/generator.go
index e42d4ef..5c8e17c 100644
--- a/generator/generator.go
+++ b/generator/generator.go
@@ -192,13 +192,13 @@ func (g *generator) CallMethod(
 	if definition.Source != nil {
 		params = append(params, sourceID.Code.Clone())
 
-		if !source.AssignableTo(definition.Source) {
+		if !source.AssignableTo(definition.Source) && !definition.TypeParams {
 			cause := fmt.Sprintf("Method source type mismatches with conversion source: %s != %s", definition.Source.String, source.String)
 			return nil, nil, formatErr(cause)
 		}
 	}
 
-	if !definition.Target.AssignableTo(target) {
+	if !definition.Target.AssignableTo(target) && !definition.TypeParams {
 		cause := fmt.Sprintf("Method return type mismatches with target: %s != %s", definition.Target.String, target.String)
 		return nil, nil, formatErr(cause)
 	}
diff --git a/method/definition.go b/method/definition.go
index 211c06d..a94e04d 100644
--- a/method/definition.go
+++ b/method/definition.go
@@ -20,6 +20,7 @@ func (m *Definition) Signature() xtype.Signature {
 type Parameters struct {
 	ReturnError          bool
 	SelfAsFirstParameter bool
+	TypeParams           bool
 	Source               *xtype.Type
 	Target               *xtype.Type
 }
diff --git a/method/parse.go b/method/parse.go
index bdf5142..142d095 100644
--- a/method/parse.go
+++ b/method/parse.go
@@ -20,9 +20,10 @@ type ParseOpts struct {
 	Converter         types.Type
 	OutputPackagePath string
 
-	ErrorPrefix  string
-	ConvFunction bool
-	Params       ParamType
+	ErrorPrefix     string
+	Params          ParamType
+	ConvFunction    bool
+	AllowTypeParams bool
 }
 
 // Parse parses an function into a Definition.
@@ -62,10 +63,15 @@ func Parse(obj types.Object, opts *ParseOpts) (*Definition, error) {
 		Parameters: Parameters{
 			ReturnError: returnError,
 			Target:      xtype.TypeOf(sig.Results().At(0).Type()),
+			TypeParams:  sig.TypeParams().Len() > 0,
 		},
 		Name: fn.Name(),
 	}
 
+	if methodDef.TypeParams && !opts.AllowTypeParams {
+		return nil, formatErr("must not be generic")
+	}
+
 	if opts.ConvFunction {
 		methodDef.Call = jen.Id(xtype.ThisVar).Dot(fn.Name())
 	} else {
diff --git a/scenario/extend_generic.yml b/scenario/extend_generic.yml
new file mode 100644
index 0000000..5161d25
--- /dev/null
+++ b/scenario/extend_generic.yml
@@ -0,0 +1,24 @@
+input:
+    input.go: |
+        package structs
+
+        // goverter:converter
+        // goverter:extend Extend
+        type Converter interface {
+            Convert(source Input) (Output, error)
+        }
+
+        func Extend[T any](T) int {
+            return 0
+        }
+        type Input struct { Age string }
+        type Output struct { Age int }
+error: |-
+    error parsing 'goverter:extend' at
+        @workdir/input.go:5:1
+        github.com/jmattheis/goverter/execution.Converter
+
+    error parsing type:
+        func github.com/jmattheis/goverter/execution.Extend[T any](T) int
+
+    must not be generic
diff --git a/scenario/map_custom_generic.yml b/scenario/map_custom_generic.yml
new file mode 100644
index 0000000..06acdf5
--- /dev/null
+++ b/scenario/map_custom_generic.yml
@@ -0,0 +1,31 @@
+input:
+    input.go: |
+        package structs
+
+        // goverter:converter
+        type Converter interface {
+            // goverter:map ID | ZeroIfNil
+            Convert(source Input) Output
+        }
+
+        type Input struct { ID *int }
+        type Output struct { ID int }
+        func ZeroIfNil[T any](*T) T {
+            var t T
+            return t
+        }
+success:
+    - generated/generated.go: |
+        // Code generated by github.com/jmattheis/goverter, DO NOT EDIT.
+
+        package generated
+
+        import execution "github.com/jmattheis/goverter/execution"
+
+        type ConverterImpl struct{}
+
+        func (c *ConverterImpl) Convert(source execution.Input) execution.Output {
+        	var structsOutput execution.Output
+        	structsOutput.ID = execution.ZeroIfNil(source.ID)
+        	return structsOutput
+        }
diff --git a/xtype/type.go b/xtype/type.go
index 05a0276..f552feb 100644
--- a/xtype/type.go
+++ b/xtype/type.go
@@ -245,6 +245,8 @@ func TypeOf(t types.Type) *Type {
 	case *types.Signature:
 		rt.Signature = true
 		rt.SignatureType = value
+	case *types.TypeParam:
+		// ignore
 	default:
 		panic("unknown types.Type " + t.String())
 	}