diff --git a/kong_test.go b/kong_test.go
index 34e89c9..b6abfd7 100644
--- a/kong_test.go
+++ b/kong_test.go
@@ -718,6 +718,22 @@ func TestMapFlagWithSliceValue(t *testing.T) {
 	assert.Equal(t, map[string][]int{"a": {1, 2}, "b": {3}}, cli.Set)
 }
 
+func TestMapFlagWithUnmappableValueType(t *testing.T) {
+	var cli struct {
+		Set map[string]any
+	}
+	_, err := mustNew(t, &cli).Parse([]string{"--set", "a=b"})
+	assert.EqualError(t, err, "--set: no mapper for value type of map[string]interface {}")
+}
+
+func TestMapFlagWithUnmappableKeyType(t *testing.T) {
+	var cli struct {
+		Set map[complex128]string
+	}
+	_, err := mustNew(t, &cli).Parse([]string{"--set", "a=b"})
+	assert.EqualError(t, err, "--set: no mapper for key type of map[complex128]string")
+}
+
 type embeddedFlags struct {
 	Embedded string
 }
diff --git a/mapper.go b/mapper.go
index 390b627..e973a5b 100644
--- a/mapper.go
+++ b/mapper.go
@@ -515,6 +515,9 @@ func mapDecoder(r *Registry) MapperFunc {
 
 			keyScanner := ScanAsType(FlagValueToken, key)
 			keyDecoder := r.ForNamedType(keyTypeName, el.Key())
+			if keyDecoder == nil {
+				return fmt.Errorf("no mapper for key type of %s", target.Type())
+			}
 			keyValue := reflect.New(el.Key()).Elem()
 			if err := keyDecoder.Decode(ctx.WithScanner(keyScanner), keyValue); err != nil {
 				return fmt.Errorf("invalid map key %q", key)
@@ -522,6 +525,9 @@ func mapDecoder(r *Registry) MapperFunc {
 
 			valueScanner := ScanAsType(FlagValueToken, value)
 			valueDecoder := r.ForNamedType(valueTypeName, el.Elem())
+			if valueDecoder == nil {
+				return fmt.Errorf("no mapper for value type of %s", target.Type())
+			}
 			valueValue := reflect.New(el.Elem()).Elem()
 			if err := valueDecoder.Decode(ctx.WithScanner(valueScanner), valueValue); err != nil {
 				return fmt.Errorf("invalid map value %q", value)