diff --git a/plugins/scenes/opengl/scene_opengl.cpp b/plugins/scenes/opengl/scene_opengl.cpp
--- a/plugins/scenes/opengl/scene_opengl.cpp
+++ b/plugins/scenes/opengl/scene_opengl.cpp
@@ -417,15 +417,15 @@
         switch (type) {
         case GL_DEBUG_TYPE_ERROR:
         case GL_DEBUG_TYPE_UNDEFINED_BEHAVIOR:
-            qCWarning(KWIN_OPENGL, "%#x: %.*s", id, length, message);
+            qCWarning(KWIN_OPENGL, "%#x: %.*s", id, length-1, message);
             break;
 
         case GL_DEBUG_TYPE_OTHER:
         case GL_DEBUG_TYPE_DEPRECATED_BEHAVIOR:
         case GL_DEBUG_TYPE_PORTABILITY:
         case GL_DEBUG_TYPE_PERFORMANCE:
         default:
-            qCDebug(KWIN_OPENGL, "%#x: %.*s", id, length, message);
+            qCDebug(KWIN_OPENGL, "%#x: %.*s", id, length-1, message);
             break;
         }
     };