diff --git a/hilti/runtime/include/types/bytes.h b/hilti/runtime/include/types/bytes.h
index 77d34a0a7..58594f1fa 100644
--- a/hilti/runtime/include/types/bytes.h
+++ b/hilti/runtime/include/types/bytes.h
@@ -15,7 +15,6 @@
 #include <hilti/rt/json-fwd.h>
 #include <hilti/rt/result.h>
 #include <hilti/rt/safe-int.h>
-#include <hilti/rt/types/string.h>
 #include <hilti/rt/types/time.h>
 #include <hilti/rt/types/vector.h>
 #include <hilti/rt/unicode.h>
@@ -445,10 +444,7 @@ class Bytes : protected std::string {
      * @param errors how to handle errors when decoding/encoding the data
      * @return an upper case version of the instance
      */
-    Bytes upper(unicode::Charset cs,
-                unicode::DecodeErrorStrategy errors = unicode::DecodeErrorStrategy::REPLACE) const {
-        return hilti::rt::string::encode(hilti::rt::string::upper(decode(cs, errors), errors), cs, errors);
-    }
+    Bytes upper(unicode::Charset cs, unicode::DecodeErrorStrategy errors = unicode::DecodeErrorStrategy::REPLACE) const;
 
     /**
      * Returns an upper-case version of the instance.
@@ -457,10 +453,7 @@ class Bytes : protected std::string {
      * @param errors how to handle errors when decoding/encoding the data
      * @return a lower case version of the instance
      */
-    Bytes lower(unicode::Charset cs,
-                unicode::DecodeErrorStrategy errors = unicode::DecodeErrorStrategy::REPLACE) const {
-        return hilti::rt::string::encode(hilti::rt::string::lower(decode(cs, errors), errors), cs, errors);
-    }
+    Bytes lower(unicode::Charset cs, unicode::DecodeErrorStrategy errors = unicode::DecodeErrorStrategy::REPLACE) const;
 
     /**
      * Removes leading and/or trailing sequences of all characters of a set
diff --git a/hilti/runtime/src/tests/integer.cc b/hilti/runtime/src/tests/integer.cc
index c186b25be..a1885adb4 100644
--- a/hilti/runtime/src/tests/integer.cc
+++ b/hilti/runtime/src/tests/integer.cc
@@ -10,6 +10,7 @@
 #include <hilti/rt/safe-int.h>
 #include <hilti/rt/types/bytes.h>
 #include <hilti/rt/types/integer.h>
+#include <hilti/rt/types/string.h>
 #include <hilti/rt/types/tuple.h>
 
 using namespace hilti::rt;
diff --git a/hilti/runtime/src/tests/union.cc b/hilti/runtime/src/tests/union.cc
index 7b76a134b..d21a953c1 100644
--- a/hilti/runtime/src/tests/union.cc
+++ b/hilti/runtime/src/tests/union.cc
@@ -6,6 +6,7 @@
 #include <hilti/rt/doctest.h>
 #include <hilti/rt/extension-points.h>
 #include <hilti/rt/types/integer.h>
+#include <hilti/rt/types/string.h>
 #include <hilti/rt/types/union.h>
 
 using namespace hilti::rt;
diff --git a/hilti/runtime/src/types/bytes.cc b/hilti/runtime/src/types/bytes.cc
index 0139303cf..f56770bd2 100644
--- a/hilti/runtime/src/types/bytes.cc
+++ b/hilti/runtime/src/types/bytes.cc
@@ -236,6 +236,10 @@ Bytes Bytes::strip(bytes::Side side) const {
     cannot_be_reached();
 }
 
+Bytes Bytes::upper(unicode::Charset cs, unicode::DecodeErrorStrategy errors) const {
+    return string::encode(string::upper(decode(cs, errors), errors), cs, errors);
+}
+
 integer::safe<int64_t> Bytes::toInt(uint64_t base) const {
     int64_t x = 0;
     if ( hilti::rt::atoi_n(str().begin(), str().end(), base, &x) == str().end() )
@@ -310,6 +314,10 @@ double Bytes::toReal() const {
     return d;
 }
 
+Bytes Bytes::lower(unicode::Charset cs, unicode::DecodeErrorStrategy errors) const {
+    return string::encode(string::lower(decode(cs, errors), errors), cs, errors);
+}
+
 Result<Bytes> Bytes::match(const RegExp& re, unsigned int group) const {
     auto groups = re.matchGroups(*this);