diff --git a/calculator.asm b/calculator.asm
index 11ce5c2..30bdf8a 100644
--- a/calculator.asm
+++ b/calculator.asm
@@ -481,6 +481,13 @@ pushByte:
 popByte:
 	load r0, SP+0
 	load r1, SP+1
+
+	load r2, SPStart+0
+	load r3, SPStart+1
+	brneq r0, r2, popByteNoUnderflow
+	breq r1, r3, error
+
+popByteNoUnderflow:
 	cleq r0, r0, decWord
 	store SP+0, r0
 	store SP+1, r1