diff --git a/src/lib/eval/eval_messages.mes b/src/lib/eval/eval_messages.mes
index 3d86f758b4..cca2b0e19e 100644
--- a/src/lib/eval/eval_messages.mes
+++ b/src/lib/eval/eval_messages.mes
@@ -18,8 +18,3 @@ $NAMESPACE isc::dhcp
 This debug message indicates that the expression has been evaluated
 to said value. This message is mostly useful during debugging of the
 client classification expressions.
-
-% EVAL_SUBSTRING_BAD_PARAM_CONVERSION starting %1, length %2
-This debug message indicates that the parameter for the starting postion
-or length of the substring couldn't be converted to an integer.  In this
-case the substring routine returns an empty string.
diff --git a/src/lib/eval/tests/token_unittest.cc b/src/lib/eval/tests/token_unittest.cc
index 7f2d653f68..1fc8109d12 100644
--- a/src/lib/eval/tests/token_unittest.cc
+++ b/src/lib/eval/tests/token_unittest.cc
@@ -72,10 +72,12 @@ public:
     /// @param test_start The postion to start when getting a substring
     /// @param test_length The length of the substring to get
     /// @param result_string The expected result of the eval
+    /// @param should_throw The eval will throw
     void verifySubstringEval(const std::string& test_string,
                              const std::string& test_start,
                              const std::string& test_length,
-                             const std::string& result_string) {
+                             const std::string& result_string,
+                             bool should_throw = false) {
 
         // create the token
         ASSERT_NO_THROW(t_.reset(new TokenSubstring()));
@@ -86,14 +88,19 @@ public:
         values_.push(test_length);
 
         // evaluate the token
-        EXPECT_NO_THROW(t_->evaluate(*pkt4_, values_));
+        if (should_throw) {
+            EXPECT_THROW(t_->evaluate(*pkt4_, values_), EvalTypeError);
+            ASSERT_EQ(0, values_.size());
+        } else {
+            EXPECT_NO_THROW(t_->evaluate(*pkt4_, values_));
 
-        // verify results
-        ASSERT_EQ(1, values_.size());
-        EXPECT_EQ(result_string, values_.top());
+            // verify results
+            ASSERT_EQ(1, values_.size());
+            EXPECT_EQ(result_string, values_.top());
 
-        // remove result
-        values_.pop();
+            // remove result
+            values_.pop();
+        }
     }
 
     /// @todo: Add more option types here
@@ -443,13 +450,13 @@ TEST_F(TokenTest, substringStartingPosition) {
 // Check what happens if we use strings that aren't numbers for start or length
 // We should return the empty string
 TEST_F(TokenTest, substringBadParams) {
-    verifySubstringEval("foobar", "0ick", "all", "");
-    verifySubstringEval("foobar", "ick0", "all", "");
-    verifySubstringEval("foobar", "ick", "all", "");
-    verifySubstringEval("foobar", "0", "ick", "");
-    verifySubstringEval("foobar", "0", "0ick", "");
-    verifySubstringEval("foobar", "0", "ick0", "");
-    verifySubstringEval("foobar", "0", "allaboard", "");
+    verifySubstringEval("foobar", "0ick", "all", "", true);
+    verifySubstringEval("foobar", "ick0", "all", "", true);
+    verifySubstringEval("foobar", "ick", "all", "", true);
+    verifySubstringEval("foobar", "0", "ick", "", true);
+    verifySubstringEval("foobar", "0", "0ick", "", true);
+    verifySubstringEval("foobar", "0", "ick0", "", true);
+    verifySubstringEval("foobar", "0", "allaboard", "", true);
 }
 
 // lastly check that we don't get anything if the string is empty or
diff --git a/src/lib/eval/token.cc b/src/lib/eval/token.cc
index ea01f11a76..e0a50bf7cd 100644
--- a/src/lib/eval/token.cc
+++ b/src/lib/eval/token.cc
@@ -121,19 +121,21 @@ TokenSubstring::evaluate(const Pkt& /*pkt*/, ValueStack& values) {
     int length;
     try {
         start_pos = boost::lexical_cast<int>(start_str);
+    } catch (const boost::bad_lexical_cast&) {
+        isc_throw(EvalTypeError, "the parameter '" << start_str
+                  << "' for the starting postion of the substring "
+                  << "couldn't be converted to an integer.");
+    }
+    try {
         if (len_str == "all") {
             length = string_str.length();
         } else {
             length = boost::lexical_cast<int>(len_str);
         }
     } catch (const boost::bad_lexical_cast&) {
-        LOG_DEBUG(eval_logger, EVAL_DBG_TRACE,
-                  EVAL_SUBSTRING_BAD_PARAM_CONVERSION)
-            .arg(start_str)
-            .arg(len_str);
-
-        values.push("");
-        return;
+        isc_throw(EvalTypeError, "the parameter '" << len_str
+                  << "' for the length of the substring "
+                  << "couldn't be converted to an integer.");
     }
 
     const int string_length = string_str.length();
diff --git a/src/lib/eval/token.h b/src/lib/eval/token.h
index 4b264c2655..659d029b7c 100644
--- a/src/lib/eval/token.h
+++ b/src/lib/eval/token.h
@@ -38,7 +38,7 @@ typedef std::vector<TokenPtr> Expression;
 /// Evaluated values are stored as a stack of strings
 typedef std::stack<std::string> ValueStack;
 
-/// @brief EvalStackError is thrown when more or less parameters are on the
+/// @brief EvalBadStack is thrown when more or less parameters are on the
 ///        stack than expected.
 class EvalBadStack : public Exception {
 public:
@@ -46,6 +46,14 @@ public:
         isc::Exception(file, line, what) { };
 };
 
+/// @brief EvalTypeError is thrown when a value on the stack has a content
+///        with an unexpected type.
+class EvalTypeError : public Exception {
+public:
+    EvalTypeError(const char* file, size_t line, const char* what) :
+        isc::Exception(file, line, what) { };
+};
+
 /// @brief Base class for all tokens
 ///
 /// It provides an interface for all tokens and storage for string representation
@@ -241,6 +249,8 @@ public:
     /// - -1, -4  => "ooba"
     ///
     /// @throw EvalBadStack if there are less than 3 values on stack
+    /// @throw EvalTypeError if start is not a number or length a number or
+    ///        the special value "all".
     ///
     /// @param pkt (unused)
     /// @param values - stack of values (3 arguments will be popped, 1 result