diff --git a/arch/X86/X86DisassemblerDecoder.c b/arch/X86/X86DisassemblerDecoder.c
index f9403a4..b4381b5 100644
--- a/arch/X86/X86DisassemblerDecoder.c
+++ b/arch/X86/X86DisassemblerDecoder.c
@@ -483,6 +483,8 @@
 	bool hasAdSize = false;
 	bool hasOpSize = false;
 
+	//initialize to an impossible value
+	insn->necessaryPrefixLocation = insn->readerCursor - 1;
 	while (isPrefix) {
 		if (insn->mode == MODE_64BIT) {
 			// eliminate consecutive redundant REX bytes in front