diff --git a/package/gmp/gmp.mk b/package/gmp/gmp.mk
index 3b732dfa71..a077c89d92 100644
--- a/package/gmp/gmp.mk
+++ b/package/gmp/gmp.mk
@@ -24,7 +24,7 @@ GMP_CONF_OPTS += --disable-assembly
 endif
 
 # GMP needs M extension for riscv assembly
-ifeq ($(BR2_RISCV_ISA_CUSTOM_RVM),)
+ifeq ($(BR2_riscv):$(BR2_RISCV_ISA_CUSTOM_RVM),y:)
 GMP_CONF_OPTS += --disable-assembly
 endif