diff --git a/src/steamnetworkingsockets/clientlib/steamnetworkingsockets_snp.cpp b/src/steamnetworkingsockets/clientlib/steamnetworkingsockets_snp.cpp
index 2709eb5..305ff47 100644
--- a/src/steamnetworkingsockets/clientlib/steamnetworkingsockets_snp.cpp
+++ b/src/steamnetworkingsockets/clientlib/steamnetworkingsockets_snp.cpp
@@ -989,12 +989,18 @@ bool CSteamNetworkConnectionBase::ProcessPlainTextDataChunk( int usecTimeSinceLa
 				case 2: READ_24BITU( nOffset, szStopWaitingOffset ); break;
 				case 3: READ_64BITU( nOffset, szStopWaitingOffset ); break;
 			}
-			if ( nOffset >= nPktNum )
+			++nOffset;
+			int64 nMinPktNumToSendAcks = nPktNum-nOffset;
+
+			// Make sure the resulting stop value makes sense.
+			// Force the use of an unsigned comparison, since a negative stop value
+			// is also illegal.  That would be rejected below, but rejecting it here
+			// is clearer.
+			if ( (uint64)nMinPktNumToSendAcks >= (uint64)nPktNum )
 			{
 				DECODE_ERROR( "stop_waiting pktNum %llu offset %llu", nPktNum, nOffset );
 			}
-			++nOffset;
-			int64 nMinPktNumToSendAcks = nPktNum-nOffset;
+
 			if ( nMinPktNumToSendAcks == m_receiverState.m_nMinPktNumToSendAcks )
 				continue;
 			if ( nMinPktNumToSendAcks < m_receiverState.m_nMinPktNumToSendAcks )