diff --git a/dlib/optimization/optimization.h b/dlib/optimization/optimization.h
index 71aa40fb6..8595ed977 100644
--- a/dlib/optimization/optimization.h
+++ b/dlib/optimization/optimization.h
@@ -209,7 +209,9 @@ namespace dlib
         double alpha = mu*poly_min_extrap(f0, d0, f1, d1);
         alpha = put_in_range(0.1*mu, 0.9*mu, alpha);
 
-        DLIB_CASSERT(alpha < std::numeric_limits<double>::infinity(), alpha);
+        DLIB_CASSERT(alpha < std::numeric_limits<double>::infinity(), 
+                     "alpha: " << alpha << " mu: " << mu << " f0: " << f0 << " d0: " << d0 << " f1: " << f1 << " d1: " << d1
+                     );
 
         double last_alpha = 0;
         double last_val = f0;
@@ -302,12 +304,16 @@ namespace dlib
         }
 
 
-        DLIB_CASSERT(alpha < std::numeric_limits<double>::infinity(), alpha);
+        DLIB_CASSERT(alpha < std::numeric_limits<double>::infinity(), 
+                     "alpha: " << alpha << " mu: " << mu << " f0: " << f0 << " d0: " << d0 << " f1: " << f1 << " d1: " << d1
+                     );
 
         // Now do the sectioning phase from 2.6.4
         while (true)
         {
-            DLIB_CASSERT(alpha < std::numeric_limits<double>::infinity(), alpha);
+            DLIB_CASSERT(alpha < std::numeric_limits<double>::infinity(), 
+                        "alpha: " << alpha << " mu: " << mu << " f0: " << f0 << " d0: " << d0 << " f1: " << f1 << " d1: " << d1
+                     );
             double first = a + tau2*(b-a);
             double last = b - tau3*(b-a);
 
@@ -358,7 +364,7 @@ namespace dlib
         typename funct_der, 
         typename T
         >
-    double find_min_quasi_newton (
+    void find_min_quasi_newton (
         const funct& f, 
         const funct_der& der, 
         T& x, 
@@ -423,7 +429,6 @@ namespace dlib
             g.swap(g2);
         }
 
-        return f_value;
     }
 
 // ----------------------------------------------------------------------------------------
@@ -433,7 +438,7 @@ namespace dlib
         typename funct_der,
         typename T
         >
-    double find_min_conjugate_gradient (
+    void find_min_conjugate_gradient (
         const funct& f,
         const funct_der& der,
         T& x,
@@ -477,7 +482,6 @@ namespace dlib
             g.swap(g2);
         }
 
-        return f_value;
     }
 
 // ----------------------------------------------------------------------------------------
diff --git a/dlib/optimization/optimization_abstract.h b/dlib/optimization/optimization_abstract.h
index 8ac27e740..2ef989e8f 100644
--- a/dlib/optimization/optimization_abstract.h
+++ b/dlib/optimization/optimization_abstract.h
@@ -149,7 +149,7 @@ namespace dlib
         typename funct_der, 
         typename T
         >
-    double find_min_quasi_newton (
+    void find_min_quasi_newton (
         const funct& f, 
         const funct_der& der, 
         T& x, 
@@ -171,7 +171,6 @@ namespace dlib
                 - the change in f() from one iteration to the next is less than min_delta
                 - f(#x) <= min_f
             - #x == the value of x that was found to minimize f()
-            - returns f(#x) 
     !*/
 
 // ----------------------------------------------------------------------------------------
@@ -181,7 +180,7 @@ namespace dlib
         typename funct_der, 
         typename T
         >
-    double find_min_conjugate_gradient (
+    void find_min_conjugate_gradient (
         const funct& f, 
         const funct_der& der, 
         T& x, 
@@ -203,7 +202,6 @@ namespace dlib
                 - the change in f() from one iteration to the next is less than min_delta
                 - f(#x) <= min_f
             - #x == the value of x that was found to minimize f()
-            - returns f(#x) 
     !*/
 
 // ----------------------------------------------------------------------------------------