diff --git a/f3dex2.s b/f3dex2.s
index 414b942..07f7a3f 100644
--- a/f3dex2.s
+++ b/f3dex2.s
@@ -22,6 +22,24 @@
     ori dst, src, 0
 .endmacro
 
+// Prohibit macros involving slt; this silently clobbers $1. You can of course
+// manually write the slt and branch instructions if you want this behavior.
+.macro blt, ra, rb, lbl
+    .error "blt is a macro using slt, and silently clobbers $1!"
+.endmacro
+
+.macro bgt, ra, rb, lbl
+    .error "bgt is a macro using slt, and silently clobbers $1!"
+.endmacro
+
+.macro ble, ra, rb, lbl
+    .error "ble is a macro using slt, and silently clobbers $1!"
+.endmacro
+
+.macro bge, ra, rb, lbl
+    .error "bge is a macro using slt, and silently clobbers $1!"
+.endmacro
+
 // Vector macros
 .macro vcopy, dst, src
     vadd dst, src, $v0[0]