diff --git a/src/prival.c b/src/prival.c
index 224bd004..77f6cffa 100644
--- a/src/prival.c
+++ b/src/prival.c
@@ -66,6 +66,11 @@ stumpless_get_priority_string( int prival ) {
   size_t priority_string_size;
   char* priority_string;
 
+  if (facility_is_invalid(get_facility(prival)))
+    return NULL;
+  if (severity_is_invalid(get_severity(prival)))
+    return NULL;
+
   facility = stumpless_get_facility_string( get_facility( prival ) );
   severity = stumpless_get_severity_string( get_severity( prival ) );
 
diff --git a/test/function/prival.cpp b/test/function/prival.cpp
index a645bf24..51df63ae 100644
--- a/test/function/prival.cpp
+++ b/test/function/prival.cpp
@@ -132,4 +132,15 @@ namespace {
 
     free( ( void * ) result );
   }
+
+  TEST(GetPriorityString, InvalidPrivals) {
+    int prival;
+    const char *result;
+
+    prival = 0x100;
+    result = stumpless_get_priority_string( prival );
+    EXPECT_STREQ( result, NULL );
+
+    free( ( void * ) result );
+  }
 }