diff --git a/functions/functions_string.go b/functions/functions_string.go
index a2ce194..dac3f49 100644
--- a/functions/functions_string.go
+++ b/functions/functions_string.go
@@ -295,6 +295,9 @@ func (f *SubstringFunction) Execute(ctx *FunctionContext, args []interface{}) (i
 	}
 
 	strLen := int64(len(str))
+	if start < 0 {
+		start = strLen + start
+	}
 	if start < 0 || start >= strLen {
 		return "", nil
 	}
@@ -308,6 +311,10 @@ func (f *SubstringFunction) Execute(ctx *FunctionContext, args []interface{}) (i
 		return nil, err
 	}
 
+	if length < 0 {
+		return "", nil
+	}
+
 	end := start + length
 	if end > strLen {
 		end = strLen