diff --git a/perfdata/type.go b/perfdata/type.go
index 4814208..dc9202e 100644
--- a/perfdata/type.go
+++ b/perfdata/type.go
@@ -43,8 +43,6 @@ func formatNumeric(value interface{}) (string, error) {
 		return check.FormatFloat(float64(v)), nil
 	case int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64:
 		return fmt.Sprintf("%d", v), nil
-	case fmt.Stringer, string:
-		return fmt.Sprint(v), nil
 	default:
 		panic(fmt.Sprintf("unsupported type for perfdata: %T", value))
 	}
diff --git a/perfdata/type_test.go b/perfdata/type_test.go
index 6168df3..5eff5e3 100644
--- a/perfdata/type_test.go
+++ b/perfdata/type_test.go
@@ -123,14 +123,11 @@ func TestFormatNumeric(t *testing.T) {
 		},
 		{
 			Result:     "1234.567",
-			InputValue: 1234.567,
+			InputValue: float64(1234.567),
 		},
 		{
-			Result:     "1234.567",
-			InputValue: float32(1234.567),
-		},
-		{Result: "1234.567",
-			InputValue: "1234.567",
+			Result:     "3456.789",
+			InputValue: float32(3456.789),
 		},
 		{
 			Result:     "1234567890.988",