diff --git a/tests/string_test.cpp b/tests/string_test.cpp
index 4b2cca2..2a9da10 100644
--- a/tests/string_test.cpp
+++ b/tests/string_test.cpp
@@ -163,7 +163,7 @@
 template<class Character>
 class StringTestState {
  public:
-  StringTestState(size_t MAX_LEN) : MAX_LEN(MAX_LEN), align1_index_(0), align2_index_(0) {
+  explicit StringTestState(size_t MAX_LEN) : MAX_LEN(MAX_LEN), align1_index_(0), align2_index_(0) {
     int max_alignment = 64;
 
     // TODO: fix the tests to not sometimes use twice their specified "MAX_LEN".