diff --git a/Internal.h b/Internal.h
index 8b79a1e..10ddf17 100644
--- a/Internal.h
+++ b/Internal.h
@@ -858,13 +858,15 @@ struct TestDriver {
                   stderr,
                   "%p Expected %s, got %s for read of {%s} at version %" PRId64
                   "\n",
-                  this, resultToStr(results2[i]), resultToStr(results1[i]),
-                  printable(reads[i].begin).c_str(), reads[i].readVersion);
+                  (void *)this, resultToStr(results2[i]),
+                  resultToStr(results1[i]), printable(reads[i].begin).c_str(),
+                  reads[i].readVersion);
             } else {
               fprintf(stderr,
                       "%p Expected %s, got %s for read of [%s, %s) at version "
                       "%" PRId64 "\n",
-                      this, resultToStr(results2[i]), resultToStr(results1[i]),
+                      (void *)this, resultToStr(results2[i]),
+                      resultToStr(results1[i]),
                       printable(reads[i].begin).c_str(),
                       printable(reads[i].end).c_str(), reads[i].readVersion);
             }