diff --git a/lib/interpolate.c b/lib/interpolate.c
index a453a05..a9c009b 100644
--- a/lib/interpolate.c
+++ b/lib/interpolate.c
@@ -49,6 +49,14 @@ fprintf(stderr, ">>> %s ( %p %p %p %f )\n", __func__,
 				s1, s2, d, coef);
 #endif
 
+/* sanity check here ? or may be ask the
+   caller to do it himself ?
+*/
+if ( (coef < 0.0) || (coef >= 1.0) ){
+	fprintf(stderr, "%s: coef %f out of range\n", __func__, coef);
+	return -8;
+	}
+
 foo = fimg_images_not_compatible(s1, s2);
 if (foo) {
 	fprintf(stderr, "compat s1 s2 -> %d\n", foo);