Index: pstk_compat.c
===================================================================
--- pstk_compat.c	(revision 15076)
+++ pstk_compat.c	(revision 15077)
@@ -123,6 +123,8 @@
 
 	assert(pad_dia > drill_dia);
 	assert(drill_dia > 0);
+	assert(clearance >= 0);
+	assert(mask >= 0);
 
 	memset(&proto, 0, sizeof(proto));
 	memset(&tshp, 0, sizeof(tshp));
@@ -443,6 +445,10 @@
 	int n;
 	pcb_coord_t cx, cy;
 
+	assert(thickness >= 0);
+	assert(clearance >= 0);
+	assert(mask >= 0);
+
 	cx = (x1 + x2) / 2;
 	cy = (y1 + y2) / 2;