diff --git a/libsel4/sel4_arch_include/aarch32/sel4/sel4_arch/syscalls.h b/libsel4/sel4_arch_include/aarch32/sel4/sel4_arch/syscalls.h
index ff3277bdd..2bb07f060 100644
--- a/libsel4/sel4_arch_include/aarch32/sel4/sel4_arch/syscalls.h
+++ b/libsel4/sel4_arch_include/aarch32/sel4/sel4_arch/syscalls.h
@@ -17,6 +17,15 @@
 
 #define __SWINUM(x) ((x) & 0x00ffffff)
 
+#ifndef __OPTIMIZE__
+/* With no optimisations (-O0) GCC's register allocator clobbers the
+ * syscall arguments before you reach the 'swi' and you invoke the kernel
+ * incorrectly.
+ * See SELFOUR-187
+ */
+#warning you are compiling with -O0; syscalls will most likely not work
+#endif
+
 static inline void
 seL4_Send(seL4_CPtr dest, seL4_MessageInfo_t msgInfo)
 {