diff --git a/xs/src/libslic3r/Fill/FillRectilinear2.cpp b/xs/src/libslic3r/Fill/FillRectilinear2.cpp
index 69f3485bd..728926eb2 100644
--- a/xs/src/libslic3r/Fill/FillRectilinear2.cpp
+++ b/xs/src/libslic3r/Fill/FillRectilinear2.cpp
@@ -1043,7 +1043,7 @@ bool FillRectilinear2::fill_surface_by_lines(const Surface *surface, const FillP
     }
 
     // Verify the segments. If something is wrong, give up.
-#define ASSERT_OR_RETURN(CONDITION) do { assert(CONDITION); if (CONDITION) return false; } while (0)
+#define ASSERT_OR_RETURN(CONDITION) do { assert(CONDITION); if (! (CONDITION)) return false; } while (0)
     for (size_t i_seg = 0; i_seg < segs.size(); ++ i_seg) {
         SegmentedIntersectionLine &sil = segs[i_seg];
         // The intersection points have to be even.
diff --git a/xs/src/libslic3r/libslic3r.h b/xs/src/libslic3r/libslic3r.h
index 48e08c1d8..0b91f55cc 100644
--- a/xs/src/libslic3r/libslic3r.h
+++ b/xs/src/libslic3r/libslic3r.h
@@ -5,7 +5,8 @@
 #include <ostream>
 #include <iostream>
 #include <sstream>
-#include <stdint.h>
+#include <cstdio>
+#include <cstdint>
 #include <stdarg.h>
 
 #define SLIC3R_VERSION "1.3.0-dev"
@@ -71,7 +72,7 @@ inline std::string debug_out_path(const char *name, ...)
 	char buffer[2048];
 	va_list args;
 	va_start(args, name);
-	vsprintf(buffer, name, args);
+	std::vsprintf(buffer, name, args);
 	va_end(args);
 	return std::string(DEBUG_FILE_PREFIX) + std::string(buffer);
 }