diff --git a/src/libstore/nar-accessor.cc b/src/libstore/nar-accessor.cc
index d884a131e..e438b5710 100644
--- a/src/libstore/nar-accessor.cc
+++ b/src/libstore/nar-accessor.cc
@@ -86,7 +86,7 @@ struct NarAccessor : public FSAccessor
             parents.top()->start = pos;
         }
 
-        void receiveContents(unsigned char * data, unsigned int len) override
+        void receiveContents(unsigned char * data, size_t len) override
         { }
 
         void createSymlink(const Path & path, const string & target) override
diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index 51c88537e..c07e710cd 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -338,7 +338,7 @@ struct RestoreSink : ParseSink
 #endif
     }
 
-    void receiveContents(unsigned char * data, unsigned int len)
+    void receiveContents(unsigned char * data, size_t len)
     {
         writeFull(fd.get(), data, len);
     }
diff --git a/src/libutil/archive.hh b/src/libutil/archive.hh
index 57780d16a..b6e1188c4 100644
--- a/src/libutil/archive.hh
+++ b/src/libutil/archive.hh
@@ -58,7 +58,7 @@ struct ParseSink
     virtual void createRegularFile(const Path & path) { };
     virtual void isExecutable() { };
     virtual void preallocateContents(unsigned long long size) { };
-    virtual void receiveContents(unsigned char * data, unsigned int len) { };
+    virtual void receiveContents(unsigned char * data, size_t len) { };
 
     virtual void createSymlink(const Path & path, const string & target) { };
 };
@@ -77,7 +77,7 @@ struct RetrieveRegularNARSink : ParseSink
         regular = false;
     }
 
-    void receiveContents(unsigned char * data, unsigned int len)
+    void receiveContents(unsigned char * data, size_t len)
     {
         sink(data, len);
     }