diff --git a/libheif/box.cc b/libheif/box.cc
index 4c0a85afe4..e752ac4807 100644
--- a/libheif/box.cc
+++ b/libheif/box.cc
@@ -85,7 +85,7 @@ Fraction::Fraction(int64_t num, int64_t den)
 Fraction Fraction::operator+(const Fraction& b) const
 {
   if (denominator == b.denominator) {
-    int64_t n = numerator + b.numerator;
+    int64_t n = int64_t{numerator} + b.numerator;
     int64_t d = denominator;
     return Fraction{n,d};
   }
@@ -99,7 +99,7 @@ Fraction Fraction::operator+(const Fraction& b) const
 Fraction Fraction::operator-(const Fraction& b) const
 {
   if (denominator == b.denominator) {
-    int64_t n = numerator - b.numerator;
+    int64_t n = int64_t{numerator} - b.numerator;
     int64_t d = denominator;
     return Fraction{n,d};
   }