diff --git a/sexp.c b/sexp.c
index 9ead8341..06e5c3ef 100644
--- a/sexp.c
+++ b/sexp.c
@@ -2091,6 +2091,7 @@ int sexp_maybe_block_port (sexp ctx, sexp in, int forcep) {
           && (((c = sexp_read_char(ctx, in)) == EOF)
               && sexp_port_stream(in)
               && ferror(sexp_port_stream(in)) && (errno == EAGAIN))) {
+        clearerr(sexp_port_stream(in));
         f = sexp_global(ctx, SEXP_G_THREADS_BLOCKER);
         if (sexp_opcodep(f)) {
           ((sexp_proc2)sexp_opcode_func(f))(ctx, f, 1, in);