diff --git a/common/fmatrix.hh b/common/fmatrix.hh
index 0241ca13c03ad2a1cde9c59c487ec7b1ffd977e8..9c09b69978c07b8079ac7d3e625fa0db619fa2e3 100644
--- a/common/fmatrix.hh
+++ b/common/fmatrix.hh
@@ -130,7 +130,7 @@ namespace Dune {
 
     /** \brief Constructor initializing the whole matrix with a scalar
      */
-    FieldMatrix (const K& k)
+    explicit FieldMatrix (const K& k)
     {
       for (size_type i=0; i<n; i++) p[i] = k;
     }
@@ -943,7 +943,7 @@ namespace Dune {
 
     /** \brief Constructor initializing the whole matrix with a scalar
      */
-    FieldMatrix (const K& k)
+    explicit FieldMatrix (const K& k)
     {
       a = k;
     }