diff --git a/internal/filter/exclude.go b/internal/filter/exclude.go
index 48ecdfddf..1d3042a58 100644
--- a/internal/filter/exclude.go
+++ b/internal/filter/exclude.go
@@ -39,11 +39,12 @@ func RejectByPattern(patterns []string, warnf func(msg string, args ...interface
 
 // RejectByInsensitivePattern is like RejectByPattern but case insensitive.
 func RejectByInsensitivePattern(patterns []string, warnf func(msg string, args ...interface{})) RejectByNameFunc {
+	lowerPatterns := make([]string, len(patterns))
 	for index, path := range patterns {
-		patterns[index] = strings.ToLower(path)
+		lowerPatterns[index] = strings.ToLower(path)
 	}
 
-	rejFunc := RejectByPattern(patterns, warnf)
+	rejFunc := RejectByPattern(lowerPatterns, warnf)
 	return func(item string) bool {
 		return rejFunc(strings.ToLower(item))
 	}
diff --git a/internal/filter/include.go b/internal/filter/include.go
index e6eefe6b9..749139f6e 100644
--- a/internal/filter/include.go
+++ b/internal/filter/include.go
@@ -92,11 +92,12 @@ func IncludeByPattern(patterns []string, warnf func(msg string, args ...interfac
 // IncludeByInsensitivePattern returns a IncludeByNameFunc which includes files that match
 // one of the patterns, ignoring the casing of the filenames.
 func IncludeByInsensitivePattern(patterns []string, warnf func(msg string, args ...interface{})) IncludeByNameFunc {
+	lowerPatterns := make([]string, len(patterns))
 	for index, path := range patterns {
-		patterns[index] = strings.ToLower(path)
+		lowerPatterns[index] = strings.ToLower(path)
 	}
 
-	includeFunc := IncludeByPattern(patterns, warnf)
+	includeFunc := IncludeByPattern(lowerPatterns, warnf)
 	return func(item string) (matched bool, childMayMatch bool) {
 		return includeFunc(strings.ToLower(item))
 	}