diff --git a/testcrypt.c b/testcrypt.c
index 8fd51d2d..37eb6869 100644
--- a/testcrypt.c
+++ b/testcrypt.c
@@ -512,6 +512,7 @@ static void return_val_string_asciz(strbuf *out, char *s)
             return_##type_name(out, ptr);                               \
     }
 
+NULLABLE_RETURN_WRAPPER(val_string, strbuf *)
 NULLABLE_RETURN_WRAPPER(val_string_asciz, char *)
 NULLABLE_RETURN_WRAPPER(val_cipher, ssh_cipher *)
 NULLABLE_RETURN_WRAPPER(val_hash, ssh_hash *)
@@ -751,11 +752,14 @@ static RSAKey *rsa_new(void)
 strbuf *rsa_ssh1_encrypt_wrapper(ptrlen input, RSAKey *key)
 {
     /* Fold the boolean return value in C into the string return value
-     * for this purpose, by returning the empty string on failure */
+     * for this purpose, by returning NULL on failure */
     strbuf *sb = strbuf_new();
     put_datapl(sb, input);
-    if (!rsa_ssh1_encrypt(sb->u, sb->len, key))
-        sb->len = 0;
+    put_padding(sb, key->bytes - input.len, 0);
+    if (!rsa_ssh1_encrypt(sb->u, input.len, key)) {
+        strbuf_free(sb);
+        return NULL;
+    }
     return sb;
 }
 #define rsa_ssh1_encrypt rsa_ssh1_encrypt_wrapper
diff --git a/testcrypt.h b/testcrypt.h
index 61a8e538..ded05d48 100644
--- a/testcrypt.h
+++ b/testcrypt.h
@@ -207,7 +207,7 @@ FUNC1(val_rsakex, get_rsa_ssh1_priv_agent, val_string_binarysource)
 FUNC0(val_rsa, rsa_new)
 FUNC3(void, get_rsa_ssh1_pub, val_string_binarysource, val_rsa, rsaorder)
 FUNC2(void, get_rsa_ssh1_priv, val_string_binarysource, val_rsa)
-FUNC2(val_string, rsa_ssh1_encrypt, val_string_ptrlen, val_rsa)
+FUNC2(opt_val_string, rsa_ssh1_encrypt, val_string_ptrlen, val_rsa)
 FUNC2(val_mpint, rsa_ssh1_decrypt, val_mpint, val_rsa)
 FUNC2(val_string, rsa_ssh1_decrypt_pkcs1, val_mpint, val_rsa)
 FUNC1(val_string_asciz, rsastr_fmt, val_rsa)