diff --git a/wolfcrypt/src/integer.c b/wolfcrypt/src/integer.c
index 3c86e665a..bd8aadb21 100644
--- a/wolfcrypt/src/integer.c
+++ b/wolfcrypt/src/integer.c
@@ -966,8 +966,8 @@ int wolfcrypt_mp_invmod(mp_int * a, mp_int * b, mp_int * c)
 int mp_invmod (mp_int * a, mp_int * b, mp_int * c)
 #endif
 {
-  /* b cannot be negative */
-  if (b->sign == MP_NEG || mp_iszero(b) == MP_YES) {
+  /* b cannot be negative or zero, and can not divide by 0 (1/a mod b) */
+  if (b->sign == MP_NEG || mp_iszero(b) == MP_YES || mp_iszero(a) == MP_YES) {
     return MP_VAL;
   }
 
diff --git a/wolfcrypt/src/tfm.c b/wolfcrypt/src/tfm.c
index 1f875d0cd..39323819c 100644
--- a/wolfcrypt/src/tfm.c
+++ b/wolfcrypt/src/tfm.c
@@ -1023,6 +1023,11 @@ int fp_invmod(fp_int *a, fp_int *b, fp_int *c)
 #endif
   int     neg;
 
+  /* [modified] sanity check on "a" */
+  if (fp_iszero(a) == FP_YES) {
+    return FP_VAL; /* can not divide by 0 here */
+  }
+
   /* 2. [modified] b must be odd   */
   if (fp_iseven (b) == FP_YES) {
     return fp_invmod_slow(a,b,c);