diff --git a/src/test-circle.cc b/src/test-circle.cc
index 2a6e7fc147965e24489b9e2d1ba4a53aa4ecdbee..a7f308e41c490e232520826d90bd50b9bda825a9 100644
--- a/src/test-circle.cc
+++ b/src/test-circle.cc
@@ -62,7 +62,7 @@ int main() {
 
   double const intervals = 10000;
   for (size_t i = 0; i < intervals; ++i) {
-    double alpha = i / (double)intervals * 2 * M_PI;
+    double const alpha = i / (double)intervals * 2 * M_PI;
     SmallVector x;
     x[0] = scale * std::sin(alpha);
     x[1] = scale * std::cos(alpha);