diff --git a/passphrase_cache.cc b/passphrase_cache.cc
index 210a94e..7e96a87 100644
--- a/passphrase_cache.cc
+++ b/passphrase_cache.cc
@@ -51,12 +51,12 @@ namespace pEp {
         
         if (_cache.empty()) {
             _which = _cache.end();
-            throw std::underflow_error("empty passphrase cache");
+            throw Empty();
         }
 
         if (_which == _cache.begin()) {
             _which = _cache.end();
-            throw std::underflow_error("out of passphrases");
+            throw Exhausted();
         }
 
         --_which;
diff --git a/passphrase_cache.hh b/passphrase_cache.hh
index 22334f9..42ffb32 100644
--- a/passphrase_cache.hh
+++ b/passphrase_cache.hh
@@ -31,6 +31,11 @@ namespace pEp {
         cache::iterator _which;
 
     public:
+        struct Empty : public std::underflow_error {
+            Empty() : std::underflow_error("passphrase cache empty") { }
+        struct Exhausted : public std::underflow_error {
+            Exhausted() : std::underflow_error("out of passphrases") { }
+
         PassphraseCache(int max_size=20, duration timeout =
                 std::chrono::minutes(10)) : _max_size(max_size),
                     _timeout(timeout), _which(_cache.end()) { }