diff --git a/source/utf8/checked.h b/source/utf8/checked.h
index a1d2035..9cb8d2c 100644
--- a/source/utf8/checked.h
+++ b/source/utf8/checked.h
@@ -161,13 +161,17 @@ namespace utf8
 
     template <typename octet_iterator>
     uint32_t prior(octet_iterator& it, octet_iterator start)
-    {
-        octet_iterator end = it;
+    {
+        // can't do much if it == start
+        if (it == start)
+            throw not_enough_room();
+
+        octet_iterator end = it;
+        // Go back until we hit either a lead octet or start
         while (internal::is_trail(*(--it)))
-            if (it < start)
+            if (it == start)
                 throw invalid_utf8(*it); // error - no lead byte in the sequence
-        octet_iterator temp = it;
-        return next(temp, end);
+        return peek_next(it, end);
     }
 
     /// Deprecated in versions that include "prior"
diff --git a/test_drivers/negative/negative.cpp b/test_drivers/negative/negative.cpp
index ee28257..0497945 100644
--- a/test_drivers/negative/negative.cpp
+++ b/test_drivers/negative/negative.cpp
@@ -22,8 +22,8 @@ int main(int argc, char** argv)
     // Open the test file
     ifstream fs8(test_file_path.c_str());
     if (!fs8.is_open()) {
-    cout << "Could not open " << test_file_path << endl;
-    return 0;
+        cout << "Could not open " << test_file_path << endl;
+        return 0;
     }
 
     // Read it line by line
@@ -32,20 +32,20 @@ int main(int argc, char** argv)
     while (!fs8.eof()) {
         string line;
         while ((byte = static_cast<char>(fs8.get())) != '\n' && !fs8.eof())
-	    line.push_back(byte);
+            line.push_back(byte);
 
         line_count++;
-	// Print out lines that contain invalid UTF-8
-	if (!is_valid(line.begin(), line.end())) {
-	   const unsigned* u = find(INVALID_LINES, INVALID_LINES_END, line_count);
-	   if (u == INVALID_LINES_END)
-	       cout << "Unexpected invalid utf-8 at line " << line_count << '\n';
+        // Print out lines that contain invalid UTF-8
+        if (!is_valid(line.begin(), line.end())) {
+            const unsigned* u = find(INVALID_LINES, INVALID_LINES_END, line_count);
+            if (u == INVALID_LINES_END)
+                cout << "Unexpected invalid utf-8 at line " << line_count << '\n';
 
-           // try fixing it:
-           string fixed_line;
-           replace_invalid(line.begin(), line.end(), back_inserter(fixed_line));
-           if (!is_valid(fixed_line.begin(), fixed_line.end()))
-               cout << "replace_invalid() resulted in an invalid utf-8 at line " << line_count << '\n';
+            // try fixing it:
+            string fixed_line;
+            replace_invalid(line.begin(), line.end(), back_inserter(fixed_line));
+            if (!is_valid(fixed_line.begin(), fixed_line.end()))
+                cout << "replace_invalid() resulted in an invalid utf-8 at line " << line_count << '\n';
         }
     }
 }