diff --git a/coverage.txt b/coverage.txt
deleted file mode 100644
index e69de29b..00000000
diff --git a/go.mod b/go.mod
index 1bfb6b3f..4cda1cf1 100644
--- a/go.mod
+++ b/go.mod
@@ -1,5 +1,7 @@
 module github.com/go-python/gpython
 
+go 1.12
+
 require (
 	github.com/gopherjs/gopherwasm v1.0.0 // indirect
 	github.com/peterh/liner v1.1.0
diff --git a/py/string.go b/py/string.go
index 83440f4b..b985621d 100644
--- a/py/string.go
+++ b/py/string.go
@@ -176,7 +176,7 @@ func init() {
 		}
 		if len(args) > 1 {
 			if s, ok := args[1].(Int); ok {
-				selfStr = selfStr[s:len(selfStr)]
+				selfStr = selfStr[s:]
 			}
 		}