diff --git a/Makefile b/Makefile
index 13e55a720..6c96f726a 100644
--- a/Makefile
+++ b/Makefile
@@ -129,6 +129,7 @@ SAIL_COQ_SRCS  = $(addprefix model/,$(SAIL_ARCH_SRCS) $(SAIL_SEQ_INST_SRCS) $(SA
 
 PLATFORM_OCAML_SRCS = $(addprefix ocaml_emulator/,platform.ml platform_impl.ml softfloat.ml riscv_ocaml_sim.ml)
 
+SAIL_FLAGS += --require-version 0.18
 SAIL_FLAGS += -dno_cast
 SAIL_DOC_FLAGS ?= -doc_embed plain
 
diff --git a/README.md b/README.md
index 25057078d..d6259c6fc 100644
--- a/README.md
+++ b/README.md
@@ -284,6 +284,8 @@ will build the 64-bit OCaml simulator in
 `ocaml_emulator/riscv_ocaml_sim_RV64` and the C simulator in
 `c_emulator/riscv_sim_RV64`.
 
+If you get an error message saying `sail: unknown option '--require-version'.` it's because your Sail compiler is too old. You need version 0.18 or later.
+
 One can build either the RV32 or the RV64 model by specifying
 `ARCH=RV32` or `ARCH=RV64` on the `make` line, and using the matching
 target suffix.  RV64 is built by default, but the RV32 model can be