diff --git a/mk/tests.mk b/mk/tests.mk
index ef38abcab65..78f5ac11f06 100644
--- a/mk/tests.mk
+++ b/mk/tests.mk
@@ -590,7 +590,7 @@ TEST_SREQ$(1)_T_$(2)_H_$(3) = \
 
 # The tests select when to use debug configuration on their own;
 # remove directive, if present, from CFG_RUSTC_FLAGS (issue #7898).
-CTEST_RUSTC_FLAGS := $$(subst -C debug-assertions,,$$(CFG_RUSTC_FLAGS))
+CTEST_RUSTC_FLAGS := $$(subst -C debug-assertions,,$$(subst -C debug-assertions=on,,$$(CFG_RUSTC_FLAGS)))
 
 # The tests cannot be optimized while the rest of the compiler is optimized, so
 # filter out the optimization (if any) from rustc and then figure out if we need