Index: source/blender/editors/space_view3d/view3d_fly.c
===================================================================
--- source/blender/editors/space_view3d/view3d_fly.c	(revision 45256)
+++ source/blender/editors/space_view3d/view3d_fly.c	(working copy)
@@ -987,8 +987,11 @@
 					/* define dvec, view direction vector */
 					dvec_tmp[0] = dvec_tmp[1] = dvec_tmp[2] = 0.0f;
 					/* move along the current axis */
-					dvec_tmp[fly->axis] = 1.0f;
 
+					if (fly->axis != -1) {
+						dvec_tmp[fly->axis] = 1.0f;
+					}
+
 					mul_m3_v3(mat, dvec_tmp);
 				}
 				else {