diff --git a/dlib/svm/structural_svm_problem.h b/dlib/svm/structural_svm_problem.h
index 31ca5f56d..65e90176f 100644
--- a/dlib/svm/structural_svm_problem.h
+++ b/dlib/svm/structural_svm_problem.h
@@ -18,7 +18,7 @@ namespace dlib
 
     template <
         typename matrix_type,
-        typename feature_vector_type_ 
+        typename feature_vector_type_ = matrix_type
         >
     class structural_svm_problem : public oca_problem<matrix_type> 
     {
diff --git a/dlib/svm/structural_svm_problem_abstract.h b/dlib/svm/structural_svm_problem_abstract.h
index f907d7ad8..de871ad48 100644
--- a/dlib/svm/structural_svm_problem_abstract.h
+++ b/dlib/svm/structural_svm_problem_abstract.h
@@ -14,7 +14,7 @@ namespace dlib
 
     template <
         typename matrix_type,
-        typename feature_vector_type_ 
+        typename feature_vector_type_ = matrix_type
         >
     class structural_svm_problem : public oca_problem<matrix_type> 
     {