diff --git a/int64.c b/int64.c
index b1c986ce..a6a98f31 100644
--- a/int64.c
+++ b/int64.c
@@ -116,7 +116,7 @@ uint64 uint64_shift_left(uint64 x, int shift)
     return x;
 }
 
-uint64 uint64_from_decimal(char *str)
+uint64 uint64_from_decimal(const char *str)
 {
     uint64 ret;
     ret.hi = ret.lo = 0;
diff --git a/int64.h b/int64.h
index 17122974..fb50fbaf 100644
--- a/int64.h
+++ b/int64.h
@@ -21,7 +21,7 @@ uint64 uint64_subtract(uint64 x, uint64 y);
 double uint64_to_double(uint64 x);
 uint64 uint64_shift_right(uint64 x, int shift);
 uint64 uint64_shift_left(uint64 x, int shift);
-uint64 uint64_from_decimal(char *str);
+uint64 uint64_from_decimal(const char *str);
 
 void BinarySink_put_uint64(BinarySink *, uint64);
 uint64 BinarySource_get_uint64(BinarySource *);