diff --git a/compare_args.cpp b/compare_args.cpp
index 666e88b..fd11243 100644
--- a/compare_args.cpp
+++ b/compare_args.cpp
@@ -240,7 +240,7 @@ bool CompareArgs::parse_args(const int argc, char **argv)
 
     if (not image_a_ or not image_b_)
     {
-        error_string_ = "Not enough image files specified\n";
+        std::cerr << "Not enough image files specified\n";
         return false;
     }
 
diff --git a/compare_args.h b/compare_args.h
index f19bc6e..fdd1193 100644
--- a/compare_args.h
+++ b/compare_args.h
@@ -60,8 +60,6 @@ class CompareArgs
     // How many pixels different to ignore.
     unsigned int threshold_pixels_;
 
-    std::string error_string_;
-
     // How much color to use in the metric.
     // 0.0 is the same as luminance_only_ = true,
     // 1.0 means full strength.
diff --git a/metric.cpp b/metric.cpp
index 5432af4..d6bd275 100644
--- a/metric.cpp
+++ b/metric.cpp
@@ -194,13 +194,14 @@ static unsigned int adaptation(const float num_one_degree_pixels)
 }
 
 
-bool yee_compare(CompareArgs &args,
+bool yee_compare(const CompareArgs &args,
+                 std::string &output_error_string,
                  std::ostream *output_verbose)
 {
     if ((args.image_a_->get_width()  != args.image_b_->get_width()) or
         (args.image_a_->get_height() != args.image_b_->get_height()))
     {
-        args.error_string_ = "Image dimensions do not match\n";
+        output_error_string = "Image dimensions do not match\n";
         return false;
     }
 
@@ -219,7 +220,7 @@ bool yee_compare(CompareArgs &args,
     }
     if (identical)
     {
-        args.error_string_ = "Images are binary identical\n";
+        output_error_string = "Images are binary identical\n";
         return true;
     }
 
@@ -425,23 +426,23 @@ bool yee_compare(CompareArgs &args,
     {
         args.image_difference_->write_to_file(args.image_difference_->get_name());
 
-        args.error_string_ += "Wrote difference image to ";
-        args.error_string_ += args.image_difference_->get_name();
-        args.error_string_ += "\n";
+        output_error_string += "Wrote difference image to ";
+        output_error_string += args.image_difference_->get_name();
+        output_error_string += "\n";
     }
 
     if (pixels_failed < args.threshold_pixels_)
     {
-        args.error_string_ = "Images are perceptually indistinguishable\n";
-        args.error_string_ += different;
+        output_error_string = "Images are perceptually indistinguishable\n";
+        output_error_string += different;
         return true;
     }
 
-    args.error_string_ = "Images are visibly different\n";
-    args.error_string_ += different;
+    output_error_string = "Images are visibly different\n";
+    output_error_string += different;
     if (args.sum_errors_)
     {
-        args.error_string_ += error_sum_buff;
+        output_error_string += error_sum_buff;
     }
 
     return false;
diff --git a/metric.h b/metric.h
index 80bf44d..78834cb 100644
--- a/metric.h
+++ b/metric.h
@@ -21,6 +21,7 @@ Place, Suite 330, Boston, MA 02111-1307 USA
 #define PERCEPTUALDIFF_METRIC_H
 
 #include <ostream>
+#include <string>
 
 
 class CompareArgs;
@@ -29,7 +30,8 @@ class CompareArgs;
 // Image comparison metric using Yee's method.
 // References: A Perceptual Metric for Production Testing, Hector Yee, Journal
 // of Graphics Tools 2004
-bool yee_compare(CompareArgs &args,
+bool yee_compare(const CompareArgs &args,
+                 std::string &output_error_string,
                  std::ostream *output_verbose=NULL);
 
 #endif
diff --git a/perceptualdiff.cpp b/perceptualdiff.cpp
index f54c632..2bec94b 100644
--- a/perceptualdiff.cpp
+++ b/perceptualdiff.cpp
@@ -39,7 +39,6 @@ int main(const int argc, char **const argv)
     {
         if (not args.parse_args(argc, argv))
         {
-            std::cerr << args.error_string_;
             return EXIT_FAILURE;
         }
         else
@@ -50,18 +49,20 @@ int main(const int argc, char **const argv)
             }
         }
 
+        std::string error_string;
         const auto passed = yee_compare(args,
+                                        error_string,
                                         args.verbose_ ? &std::cout : NULL);
         if (passed)
         {
             if (args.verbose_)
             {
-                std::cout << "PASS: " << args.error_string_;
+                std::cout << "PASS: " << error_string;
             }
         }
         else
         {
-            std::cout << "FAIL: " << args.error_string_;
+            std::cout << "FAIL: " << error_string;
         }
 
         return passed ? EXIT_SUCCESS : EXIT_FAILURE;