diff --git a/test/test_util.cpp b/test/test_util.cpp
index 98d1c42..4673e51 100644
--- a/test/test_util.cpp
+++ b/test/test_util.cpp
@@ -176,6 +176,10 @@
     prefix = toCidr(AF_INET6, mask);
     EXPECT_EQ(prefix, 126);
 
+    mask = "ffff:ffff:ffff:ffff::";
+    prefix = toCidr(AF_INET6, mask);
+    EXPECT_EQ(prefix, 64);
+
     // Invalid Masks
     mask = "ffff:0fff::";
     prefix = toCidr(AF_INET6, mask);
diff --git a/util.cpp b/util.cpp
index eb3bf5f..25d2838 100644
--- a/util.cpp
+++ b/util.cpp
@@ -59,7 +59,21 @@
                             entry("SUBNETMASK=%s", subnetMask.c_str()));
             return 0;
         }
-        trailingZeroes = __builtin_ctz(subnet.s6_addr32[lv]);
+
+        // The __builtin_ctz function returns -1 when the value is 0 on arm.
+        // GCC's doc specifies that:
+        //  If x is 0, the result is undefined.
+        //  https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Other-Builtins.html
+        // So we could not rely on the undefined behavior.
+        // Handling the 0 case specifically fixes the issue.
+        if (subnet.s6_addr32[lv] == 0)
+        {
+            trailingZeroes = 32;
+        }
+        else
+        {
+            trailingZeroes = __builtin_ctz(subnet.s6_addr32[lv]);
+        }
         zeroesFound |= trailingZeroes;
 
         if (bitsSet + trailingZeroes != 32)