diff --git a/passphrase_cache.cc b/passphrase_cache.cc
index 57bb73e..3ba6442 100644
--- a/passphrase_cache.cc
+++ b/passphrase_cache.cc
@@ -6,7 +6,7 @@ namespace pEp {
             passphrase{p, 0, PassphraseCache::cache_entry::max_len}, tp{t}
     { }
 
-    PassphraseCache::PassphraseCache(int max_size, duration timeout) :
+    PassphraseCache::PassphraseCache(size_t max_size, duration timeout) :
             _max_size{max_size}, _timeout{timeout}, _which{_cache.end()}
     { }
 
diff --git a/passphrase_cache.hh b/passphrase_cache.hh
index 471307d..97e60fa 100644
--- a/passphrase_cache.hh
+++ b/passphrase_cache.hh
@@ -25,7 +25,7 @@ namespace pEp {
 
         cache _cache;   
         std::mutex _mtx;
-        int _max_size;
+        size_t _max_size;
         duration _timeout;
 
         cache::iterator _which;
@@ -38,7 +38,7 @@ namespace pEp {
             Exhausted() : std::underflow_error("out of passphrases") { }
         };
 
-        PassphraseCache(int max_size=20, duration timeout = std::chrono::minutes(10));
+        PassphraseCache(size_t max_size=20, duration timeout = std::chrono::minutes(10));
         ~PassphraseCache() { }
         PassphraseCache(const PassphraseCache& second);
         PassphraseCache& operator=(const PassphraseCache& second);