diff --git a/crypto/bn/bn_exp.c b/crypto/bn/bn_exp.c
index 51c8282593ee366f886b471fa20665ac8c6207b2..f7e7ced2ca0840973aa205f6323b7eccf4ee9f49 100644
--- a/crypto/bn/bn_exp.c
+++ b/crypto/bn/bn_exp.c
@@ -240,11 +240,6 @@ int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
 		ret = BN_one(r);
 		return ret;
 		}
-	if (BN_is_zero(a))
-		{
-		ret = BN_zero(r);
-		return ret;
-		}
 
 	BN_CTX_start(ctx);
 	if ((aa = BN_CTX_get(ctx)) == NULL) goto err;
@@ -256,6 +251,11 @@ int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
 	ts=1;
 
 	if (!BN_nnmod(&(val[0]),a,m,ctx)) goto err;		/* 1 */
+	if (BN_is_zero(&(val[0])))
+		{
+		ret = BN_zero(r);
+		goto err;
+		}
 
 	window = BN_window_bits_for_exponent_size(bits);
 	if (window > 1)
@@ -365,11 +365,7 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
 		ret = BN_one(rr);
 		return ret;
 		}
-	if (BN_is_zero(a))
-		{
-		ret = BN_zero(rr);
-		return ret;
-		}
+
 	BN_CTX_start(ctx);
 	d = BN_CTX_get(ctx);
 	r = BN_CTX_get(ctx);
@@ -396,6 +392,11 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
 		}
 	else
 		aa=a;
+	if (BN_is_zero(aa))
+		{
+		ret = BN_zero(rr);
+		goto err;
+		}
 	if (!BN_to_montgomery(&(val[0]),aa,mont,ctx)) goto err; /* 1 */
 
 	window = BN_window_bits_for_exponent_size(bits);
@@ -632,11 +633,6 @@ int BN_mod_exp_simple(BIGNUM *r,
 		ret = BN_one(r);
 		return ret;
 		}
-	if (BN_is_zero(a))
-		{
-		ret = BN_one(r);
-		return ret;
-		}
 
 	BN_CTX_start(ctx);
 	if ((d = BN_CTX_get(ctx)) == NULL) goto err;
@@ -644,6 +640,11 @@ int BN_mod_exp_simple(BIGNUM *r,
 	BN_init(&(val[0]));
 	ts=1;
 	if (!BN_nnmod(&(val[0]),a,m,ctx)) goto err;		/* 1 */
+	if (BN_is_zero(&(val[0])))
+		{
+		ret = BN_one(r);
+		return ret;
+		}
 
 	window = BN_window_bits_for_exponent_size(bits);
 	if (window > 1)
diff --git a/crypto/bn/bn_exp2.c b/crypto/bn/bn_exp2.c
index 56f1c959bddce4cc363f0d9d65bb77f57de4be42..4b41dae07896ccf0f844b03839e0651e67b830ff 100644
--- a/crypto/bn/bn_exp2.c
+++ b/crypto/bn/bn_exp2.c
@@ -144,11 +144,6 @@ int BN_mod_exp2_mont(BIGNUM *rr, const BIGNUM *a1, const BIGNUM *p1,
 		ret = BN_one(rr);
 		return ret;
 		}
-	if (BN_is_zero(a1) || BN_is_zero(a2))
-		{
-		ret = BN_zero(rr);
-		return ret;
-		}
 	
 	bits=(bits1 > bits2)?bits1:bits2;
 
@@ -173,7 +168,7 @@ int BN_mod_exp2_mont(BIGNUM *rr, const BIGNUM *a1, const BIGNUM *p1,
 	 */
 	BN_init(&val1[0]);
 	ts1=1;
-	if (BN_ucmp(a1,m) >= 0)
+	if (a1->neg || BN_ucmp(a1,m) >= 0)
 		{
 		if (!BN_mod(&(val1[0]),a1,m,ctx))
 			goto err;
@@ -181,6 +176,12 @@ int BN_mod_exp2_mont(BIGNUM *rr, const BIGNUM *a1, const BIGNUM *p1,
 		}
 	else
 		a_mod_m = a1;
+	if (BN_is_zero(&(val1[0])))
+		{
+		ret = BN_zero(rr);
+		goto err;
+		}
+
 	if (!BN_to_montgomery(&(val1[0]),a_mod_m,mont,ctx)) goto err;
 	if (window1 > 1)
 		{
@@ -202,7 +203,7 @@ int BN_mod_exp2_mont(BIGNUM *rr, const BIGNUM *a1, const BIGNUM *p1,
 	 */
 	BN_init(&val2[0]);
 	ts2=1;
-	if (BN_ucmp(a2,m) >= 0)
+	if (a2->neg || BN_ucmp(a2,m) >= 0)
 		{
 		if (!BN_mod(&(val2[0]),a2,m,ctx))
 			goto err;
@@ -210,6 +211,11 @@ int BN_mod_exp2_mont(BIGNUM *rr, const BIGNUM *a1, const BIGNUM *p1,
 		}
 	else
 		a_mod_m = a2;
+	if (BN_is_zero(&(val2[0])))
+		{
+		ret = BN_zero(rr);
+		goto err;
+		}
 	if (!BN_to_montgomery(&(val2[0]),a_mod_m,mont,ctx)) goto err;
 	if (window2 > 1)
 		{