diff --git a/libs/pbd/pbd/stacktrace.h b/libs/pbd/pbd/stacktrace.h
index cd8ce14605..daf61e88c1 100644
--- a/libs/pbd/pbd/stacktrace.h
+++ b/libs/pbd/pbd/stacktrace.h
@@ -24,6 +24,7 @@
 #ifdef HAVE_WAFBUILD
 #include "libpbd-config.h"
 #endif
+#include <cstddef>
 
 #include <iostream>
 #include <ostream>
@@ -40,7 +41,7 @@
 
 namespace PBD {
 
-	LIBPBD_API void stacktrace (std::ostream& out, int levels = 0, int start_level = 0);
+	LIBPBD_API void stacktrace (std::ostream& out, int levels = 0, size_t start_level = 0);
 	LIBPBD_API void trace_twb();
 
 template<typename T>
diff --git a/libs/pbd/stacktrace.cc b/libs/pbd/stacktrace.cc
index adab339db5..f8ba7e08b3 100644
--- a/libs/pbd/stacktrace.cc
+++ b/libs/pbd/stacktrace.cc
@@ -49,7 +49,7 @@ PBD::trace_twb ()
 #include <execinfo.h>
 
 void
-PBD::stacktrace (std::ostream& out, int levels, int start)
+PBD::stacktrace (std::ostream& out, int levels, size_t start)
 {
 	void *array[200];
 	size_t size;