diff --git a/src/imageutils.cc b/src/imageutils.cc
index 08b0492f..a1acaac0 100644
--- a/src/imageutils.cc
+++ b/src/imageutils.cc
@@ -1,7 +1,7 @@
 #include "imageutils.h"
+#include <assert.h>
 #include <string.h>
 #include <fstream>
-#include <assert>
 
 void flip_image(const unsigned char *src, unsigned char *dst, size_t pixelsize, size_t width, size_t height)
 {
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 246fe638..bb2ab06c 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -573,7 +573,7 @@ set_target_properties(throwntogethertest PROPERTIES COMPILE_FLAGS "-DENABLE_OPEN
 target_link_libraries(throwntogethertest tests-offscreen tests-cgal ${OPENCSG_LIBRARY} ${TESTS-CGAL-LIBRARIES} ${GLEW_LIBRARY} ${COCOA_LIBRARY})
 
 #
-# gui tests (simple wrappers around the GUI binary built by qmake)
+# GUI binary tests
 #
 if(APPLE)
   set(GUI_BINPATH "${CMAKE_CURRENT_SOURCE_DIR}/../OpenSCAD.app/Contents/MacOS/OpenSCAD")
@@ -583,8 +583,14 @@ else()
   set(GUI_BINPATH "${CMAKE_CURRENT_SOURCE_DIR}/../openscad")
 endif()
 
-add_executable(guicgalpngtest guicgalpngtest.cc)
-set_target_properties(guicgalpngtest PROPERTIES COMPILE_FLAGS "-DBINPATH=${GUI_BINPATH}")
+if(EXISTS "${GUI_BINPATH}")
+  add_executable(guicgalpngtest guicgalpngtest.cc)
+  set_target_properties(guicgalpngtest PROPERTIES COMPILE_FLAGS "-DBINPATH=${GUI_BINPATH}")
+  message(STATUS "Found OpenSCAD GUI binary: ${GUI_BINPATH}")
+else()
+  message(STATUS "Couldn't find the OpenSCAD GUI binary: ${GUI_BINPATH}")
+  message(FATAL_ERROR "Please build the OpenSCAD GUI binary and place it here: ${GUI_BINPATH}" )
+endif()
 
 #
 # Tags tests as disabled. This is more convenient than removing them manually