diff --git a/tests/test_assert.h b/tests/test_assert.h
index e6140cc..9d5f780 100644
--- a/tests/test_assert.h
+++ b/tests/test_assert.h
@@ -51,10 +51,10 @@
 
 // Write captured state to a log and terminate test run.
 void TestFail(const char *expval, const char *val, const char *exp,
-              const char *file, int line, const char *func = 0);
+              const char *file, int line, const char *func = nullptr);
 
 void TestEqStr(const char *expval, const char *val, const char *exp,
-               const char *file, int line, const char *func = 0);
+               const char *file, int line, const char *func = nullptr);
 
 // Workaround for `enum class` printing.
 // There is an issue with the printing of enums with a fixed underlying type.