diff --git a/gtests/freebl_gtest/mpi_unittest.cc b/gtests/freebl_gtest/mpi_unittest.cc
index 37dc1f9788..0503006f94 100644
--- a/gtests/freebl_gtest/mpi_unittest.cc
+++ b/gtests/freebl_gtest/mpi_unittest.cc
@@ -192,6 +192,39 @@ TEST_F(MPITest, MpiFixlenOctetsZero) {
   TestToFixedOctets(zero, sizeof(mp_digit) + 1);
 }
 
+TEST_F(MPITest, MpiRadixSizeNeg) {
+  char* str;
+  mp_int a;
+  mp_err rv;
+  const char* negative_edge =
+      "-5400000000000000003000000002200020090919017007777777777870000090"
+      "00000000007500443416610000000000000000000000000000000000000000000"
+      "00000000000000000000000000000000000000000000000000000000075049054"
+      "18610000800555594485440016000031555550000000000000000220030200909"
+      "19017007777777700000000000000000000000000000000000000000000000000"
+      "00000000000500000000000000000000000000004668129841661000071000000"
+      "00000000000000000000000000000000000000000000000007504434166100000"
+      "00000000000000000000000000000000000000000000000000000000000000000"
+      "00000000075049054186100008005555944854400184572169555500000000000"
+      "0000022003020090919017007777777700000000000000000000";
+
+  rv = mp_init(&a);
+  ASSERT_EQ(MP_OKAY, rv);
+  rv = mp_read_variable_radix(&a, negative_edge, 10);
+  ASSERT_EQ(MP_OKAY, rv);
+
+  const int radixSize = mp_radix_size(&a, 10);
+  ASSERT_LE(0, radixSize);
+
+  str = (char*)malloc(radixSize);
+  ASSERT_NE(nullptr, str);
+  rv = mp_toradix(&a, str, 10);
+  ASSERT_EQ(MP_OKAY, rv);
+  ASSERT_EQ(0, strcmp(negative_edge, str));
+  free(str);
+  mp_clear(&a);
+}
+
 TEST_F(MPITest, MpiFixlenOctetsVarlen) {
   std::vector<uint8_t> packed;
   for (size_t i = 0; i < sizeof(mp_digit) * 2; ++i) {
diff --git a/lib/freebl/mpi/mpi.c b/lib/freebl/mpi/mpi.c
index 1b7b171e7d..1d306696fb 100644
--- a/lib/freebl/mpi/mpi.c
+++ b/lib/freebl/mpi/mpi.c
@@ -2693,7 +2693,7 @@ mp_radix_size(mp_int *mp, int radix)
 
     bits = USED(mp) * DIGIT_BIT - 1;
 
-    return s_mp_outlen(bits, radix);
+    return SIGN(mp) + s_mp_outlen(bits, radix);
 
 } /* end mp_radix_size() */