diff --git a/AccountState.h b/AccountState.h
index c3958cff0..c321b9526 100644
--- a/AccountState.h
+++ b/AccountState.h
@@ -30,8 +30,9 @@ public:
 	uint32 getSeq() const { return mAccountSeq; }
 
 	bool charge(uint64 a) { mBalance+=a; }
-	bool credit(uint64 a) { mBalance-=a; }
+	bool credit(uint64 a) { assert(mBalance>=a); mBalance-=a; }
 	void incSeq(void) { mAccountSeq++; }
+	void decSeq(void) { assert(mAccountSeq!=0); mAccountSeq--; }
 	
 	std::vector<unsigned char> getRaw() const;
 };