diff --git a/src/backward.h b/src/backward.h
index 5128410..29beb81 100644
--- a/src/backward.h
+++ b/src/backward.h
@@ -3266,7 +3266,7 @@ class TraceResolverDarwinImpl<trace_resolver_tag::backtrace_symbol> : public Tra
         if (st.size() == 0) {
             return;
         }
-        _symbols.reset(backtrace_symbols(st.begin(), st.size()));
+        _symbols.reset(backtrace_symbols(st.begin(), (int)st.size()));
     }
 
     ResolvedTrace resolve(ResolvedTrace trace)