diff --git a/src/cmd/flux-start.c b/src/cmd/flux-start.c
index 7211f68ca3cd..c0496b350e60 100644
--- a/src/cmd/flux-start.c
+++ b/src/cmd/flux-start.c
@@ -208,6 +208,8 @@ int main (int argc, char *argv[])
         status = exec_broker (command, len, broker_path);
         break;
     case BOOTSTRAP_SELFPMI:
+        if (!optparse_hasopt (ctx.opts, "size"))
+            log_msg_exit ("--size must be specified for --bootstrap=selfpmi");
         status = start_session (command, len, broker_path);
         break;
     default:
diff --git a/t/t0001-basic.t b/t/t0001-basic.t
index 7bd96c460868..9e52f948c9e4 100755
--- a/t/t0001-basic.t
+++ b/t/t0001-basic.t
@@ -69,6 +69,9 @@ test_expect_success 'flux-start with size 2 has a peer' "
 test_expect_success 'flux-start --size=1 --bootstrap=selfpmi works' "
 	flux start ${ARGS} --size=1 --bootstrap=selfpmi /bin/true
 "
+test_expect_success 'flux-start --bootstrap=selfpmi fails (no size specified)' "
+	test_must_fail flux start ${ARGS} --bootstrap=selfpmi /bin/true
+"
 test_expect_success 'flux-start --size=1 --boostrap=pmi fails' "
 	test_must_fail flux start ${ARGS} --size=1 --bootstrap=pmi /bin/true
 "