Index: mcs/class/corlib/Mono.Math/BigInteger.cs
===================================================================
--- mcs/class/corlib/Mono.Math/BigInteger.cs	(revision 86048)
+++ mcs/class/corlib/Mono.Math/BigInteger.cs	(working copy)
@@ -1574,7 +1574,7 @@
 						uint j = 1;
 
 						// Multiply and add
-						for (; j < m.length; j++) {
+						for (; j < m.length && j < A.length; j++) {
 							c += (ulong)u_i * (ulong)*(mP++) + *(aSP++);
 							*(aDP++) = (uint)c;
 							c >>= 32;
Index: mcs/class/Mono.Security/Mono.Math/BigInteger.cs
===================================================================
--- mcs/class/Mono.Security/Mono.Math/BigInteger.cs	(revision 86048)
+++ mcs/class/Mono.Security/Mono.Math/BigInteger.cs	(working copy)
@@ -1574,7 +1574,7 @@
 						uint j = 1;
 
 						// Multiply and add
-						for (; j < m.length; j++) {
+						for (; j < m.length && j < A.length; j++) {
 							c += (ulong)u_i * (ulong)*(mP++) + *(aSP++);
 							*(aDP++) = (uint)c;
 							c >>= 32;