diff --git a/sexp2dsa.c b/sexp2dsa.c
index e7731cc6e3c44a4fd2461b3039b0b5b816853e71..357b3cf85401d05560c7ba0dceb1c3013f559df6 100644
--- a/sexp2dsa.c
+++ b/sexp2dsa.c
@@ -35,8 +35,12 @@
 
 #include <string.h>
 
-#define GET(x, l, v) \
-do { if (!nettle_mpz_set_sexp((x), (l), (v))) return 0; } while(0)
+#define GET(x, l, v)				\
+do {						\
+  if (!nettle_mpz_set_sexp((x), (l), (v))	\
+      || mpz_sgn(x) <= 0)			\
+    return 0;					\
+} while(0)
 
 /* Iterator should point past the algorithm tag, e.g.
  *
diff --git a/sexp2rsa.c b/sexp2rsa.c
index 8511140849bdaadb1b8f6d34944f520d1fd17264..c5795db3d1f71bc4bee0f6738662a9dfe9c28c94 100644
--- a/sexp2rsa.c
+++ b/sexp2rsa.c
@@ -35,8 +35,12 @@
 
 #include <string.h>
 
-#define GET(x, l, v) \
-do { if (!nettle_mpz_set_sexp((x), (l), (v))) return 0; } while(0)
+#define GET(x, l, v)				\
+do {						\
+  if (!nettle_mpz_set_sexp((x), (l), (v))	\
+      || mpz_sgn(x) <= 0)			\
+    return 0;					\
+} while(0)
 
 /* Iterator should point past the algorithm tag, e.g.
  *