diff --git a/dlib/svm/sparse_vector.h b/dlib/svm/sparse_vector.h
index 4a1b9a209..54a178a60 100644
--- a/dlib/svm/sparse_vector.h
+++ b/dlib/svm/sparse_vector.h
@@ -185,6 +185,14 @@ namespace dlib
             }
         }
 
+    // ------------------------------------------------------------------------------------
+
+        template <typename T>
+        struct has_unsigned_keys
+        {
+            static const bool value = is_unsigned_type<typename T::value_type::first_type>::value;
+        };
+
     // ------------------------------------------------------------------------------------
 
         template <typename T, typename U>
diff --git a/dlib/svm/sparse_vector_abstract.h b/dlib/svm/sparse_vector_abstract.h
index b144c7c41..5ec0798ee 100644
--- a/dlib/svm/sparse_vector_abstract.h
+++ b/dlib/svm/sparse_vector_abstract.h
@@ -182,6 +182,20 @@ namespace dlib
 
     }
 
+// ----------------------------------------------------------------------------------------
+
+    /*!A has_unsigned_keys
+
+        This is a template where has_unsigned_keys<T>::value == true when T is a
+        sparse vector that contains unsigned integral keys and false otherwise.
+    !*/
+
+    template <typename T>
+    struct has_unsigned_keys
+    {
+        static const bool value = is_unsigned_type<typename T::value_type::first_type>::value;
+    };
+
 // ----------------------------------------------------------------------------------------
 
 }
diff --git a/dlib/svm/svm_c_linear_trainer.h b/dlib/svm/svm_c_linear_trainer.h
index 55a66e019..d6495d512 100644
--- a/dlib/svm/svm_c_linear_trainer.h
+++ b/dlib/svm/svm_c_linear_trainer.h
@@ -238,6 +238,13 @@ namespace dlib
             T must be a sparse vector with an integral key type
         !*/
         {
+            typedef typename T::type sample_type;
+            // You are getting this error because you are attempting to use sparse sample vectors with
+            // the svm_c_linear_trainer object but you aren't using an unsigned integer as your key type
+            // in the sparse vectors.
+            COMPILE_TIME_ASSERT(sparse_vector::has_unsigned_keys<sample_type>::value);
+
+
             // these should be sparse samples so look over all them to find the max dimension.
             unsigned long max_dim = 0;
             for (long i = 0; i < samples.size(); ++i)
diff --git a/dlib/svm/svm_c_linear_trainer_abstract.h b/dlib/svm/svm_c_linear_trainer_abstract.h
index d6b2f7a2e..9469aff54 100644
--- a/dlib/svm/svm_c_linear_trainer_abstract.h
+++ b/dlib/svm/svm_c_linear_trainer_abstract.h
@@ -17,7 +17,8 @@ namespace dlib
     {
         /*!
             REQUIREMENTS ON K 
-                is either linear_kernel or sparse_linear_kernel
+                Is either linear_kernel or sparse_linear_kernel.  If you use a sparse_linear_kernel
+                then it must operate on sparse vectors that use unsigned integral keys.
 
             WHAT THIS OBJECT REPRESENTS
                 This object represents a tool for training the C formulation of