diff --git a/builtin-string.go b/builtin-string.go
index a139452..2d487ae 100644
--- a/builtin-string.go
+++ b/builtin-string.go
@@ -255,11 +255,11 @@ func ImportStringFuncs(ctx ExprContext) {
 		NewFuncParamFlag(strParamOther, PfRepeat),
 	})
 
-	ctx.RegisterFunc("strUpper", NewGolangFunctor(upperStrFunc), TypeBoolean, []ExprFuncParam{
+	ctx.RegisterFunc("strUpper", NewGolangFunctor(upperStrFunc), TypeString, []ExprFuncParam{
 		NewFuncParam(ParamSource),
 	})
 
-	ctx.RegisterFunc("strLower", NewGolangFunctor(lowerStrFunc), TypeBoolean, []ExprFuncParam{
+	ctx.RegisterFunc("strLower", NewGolangFunctor(lowerStrFunc), TypeString, []ExprFuncParam{
 		NewFuncParam(ParamSource),
 	})
 }