diff --git a/regression/cbmc/unsigned___int128/main.c b/regression/cbmc/unsigned___int128/main.c
new file mode 100644
index 00000000000..b695484a332
--- /dev/null
+++ b/regression/cbmc/unsigned___int128/main.c
@@ -0,0 +1,104 @@
+# include <stdint.h>
+
+typedef unsigned __int128 uint128_t; 
+
+typedef uint64_t limb;
+typedef uint128_t widelimb;
+
+typedef limb felem[4];
+typedef widelimb widefelem[7];
+
+felem p = {0x1FFFFFFFFFFFFFF, 
+           0xFFFFFFFFFFFFFF, 
+           0xFFFFE000000000, 
+           0x00000000000002};
+
+
+/*-
+ * Reduce seven 128-bit coefficients to four 64-bit coefficients.
+ * Requires in[i] < 2^126,
+ * ensures out[0] < 2^56, out[1] < 2^56, out[2] < 2^56, out[3] <= 2^56 + 2^16 */
+void reduce(felem out, const widefelem in)
+{
+
+    __CPROVER_assume(in[0]<(widelimb)((widelimb)1<<126));
+    __CPROVER_assume(in[1]<((widelimb)1<<126));
+    __CPROVER_assume(in[2]<((widelimb)1<<126));
+    __CPROVER_assume(in[3]<((widelimb)1<<126));
+
+    static const widelimb two127p15 = (((widelimb) 1) << 127) +
+        (((widelimb) 1) << 15);
+    static const widelimb two127m71 = (((widelimb) 1) << 127) -
+        (((widelimb) 1) << 71);
+    static const widelimb two127m71m55 = (((widelimb) 1) << 127) -
+        (((widelimb) 1) << 71) - (((widelimb) 1) << 55);
+    widelimb output[5];
+
+    /* Add 0 mod 2^224-2^96+1 to ensure all differences are positive */
+    output[0] = in[0] + two127p15;
+    output[1] = in[1] + two127m71m55;
+    output[2] = in[2] + two127m71;
+    output[3] = in[3];
+    output[4] = in[4];
+
+    /* Eliminate in[4], in[5], in[6] */
+    output[4] += in[6] >> 16;
+    output[3] += (in[6] & 0xffff) << 40;
+    output[2] -= in[6];
+
+    output[3] += in[5] >> 16;
+    output[2] += (in[5] & 0xffff) << 40;
+    output[1] -= in[5];
+
+    output[2] += output[4] >> 16;
+    output[1] += (output[4] & 0xffff) << 40;
+    output[0] -= output[4];
+
+    /* Carry 2 -> 3 -> 4 */
+    output[3] += output[2] >> 56;
+    output[2] &= 0x00ffffffffffffff;
+
+    output[4] = output[3] >> 56;
+    output[3] &= 0x00ffffffffffffff;
+
+    /* Now output[2] < 2^56, output[3] < 2^56, output[4] < 2^72 */
+
+    /* Eliminate output[4] */
+    output[2] += output[4] >> 16;
+    /* output[2] < 2^56 + 2^56 = 2^57 */
+    output[1] += (output[4] & 0xffff) << 40;
+    output[0] -= output[4];
+
+    /* Carry 0 -> 1 -> 2 -> 3 */
+    output[1] += output[0] >> 56;
+    out[0] = output[0] & 0x00ffffffffffffff;
+
+    output[2] += output[1] >> 56;
+    /* output[2] < 2^57 + 2^72 */
+    
+    assert(output[2] < (((widelimb)1)<<57)+(((widelimb)1)<<72));
+    
+    out[1] = output[1] & 0x00ffffffffffffff;
+    output[3] += output[2] >> 56;
+    /* output[3] <= 2^56 + 2^16 */
+    assert(output[3] < (((widelimb)1)<<56)+(((widelimb)1)<<16));
+
+    out[2] = output[2] & 0x00ffffffffffffff;
+
+    /*-
+     * out[0] < 2^56, out[1] < 2^56, out[2] < 2^56,
+     * out[3] <= 2^56 + 2^16 (due to final carry),
+     * so out < 2*p
+     * p[0],p[1],p[2],p[3]
+     * p = p[0] + 2**56 * p[1] ...
+     *
+     * out[3] < p[3] ||
+     * out[3] == p[3] && (
+     *
+     */
+    out[3] = output[3];
+    assert(out[3] < p[0] ||
+          (out[3] == p[0] && (
+          out[2] < p[1] || (out[2]==p[1] && (out[1]<p[2] || out[1]==p[2]
+          && (out[0]<p[3]))))));
+}
diff --git a/regression/cbmc/unsigned___int128/test.desc b/regression/cbmc/unsigned___int128/test.desc
new file mode 100644
index 00000000000..67b0294bc90
--- /dev/null
+++ b/regression/cbmc/unsigned___int128/test.desc
@@ -0,0 +1,8 @@
+CORE
+main.c
+--unsigned-overflow-check --signed-overflow-check --function reduce
+^EXIT=0$
+^SIGNAL=0$
+^VERIFICATION SUCCESSFUL$
+--
+^warning: ignoring
diff --git a/src/ansi-c/c_types.cpp b/src/ansi-c/c_types.cpp
index 1c4794e9a2c..6ba5ddaefb3 100644
--- a/src/ansi-c/c_types.cpp
+++ b/src/ansi-c/c_types.cpp
@@ -636,7 +636,7 @@ Function: gcc_unsigned_int128_type
 
 typet gcc_unsigned_int128_type()
 {
-  typet result=signedbv_typet(128);
+  typet result=unsignedbv_typet(128);
   result.set(ID_C_c_type, ID_unsigned_int128);
   return result;
 }