diff --git a/flang/include/flang/Common/uint128.h b/flang/include/flang/Common/uint128.h
--- a/flang/include/flang/Common/uint128.h
+++ b/flang/include/flang/Common/uint128.h
@@ -20,6 +20,7 @@
 #endif
 
 #include "leading-zero-bit-count.h"
+#include <cassert>
 #include <cstdint>
 #include <type_traits>
 
@@ -152,6 +153,13 @@
   }
 
   constexpr Int128 operator/(Int128 that) const {
+    assert(that != 0 && "The divisor cannot be 0!");
+    if (high_ == 0 && that.high_ == 0) {
+      return low_ / that.low_;
+    }
+    if (*this < that) {
+      return 0;
+    }
     int j{LeadingZeroes()};
     Int128 bits{*this};
     bits <<= j;
@@ -165,7 +173,7 @@
       bits <<= 1;
       quotient <<= 1;
       if (numerator >= that) {
-        ++quotient;
+        quotient |= 1;
         numerator -= that;
       }
     }
@@ -173,6 +181,13 @@
   }
 
   constexpr Int128 operator%(Int128 that) const {
+    assert(that != 0 && "The divisor cannot be 0!");
+    if (high_ == 0 && that.high_ == 0) {
+      return low_ % that.low_;
+    }
+    if (*this < that) {
+      return *this;
+    }
     int j{LeadingZeroes()};
     Int128 bits{*this};
     bits <<= j;