diff --git a/icu4c/source/common/uidna.cpp b/icu4c/source/common/uidna.cpp
index 549c93c775a..d66b576dd3c 100644
--- a/icu4c/source/common/uidna.cpp
+++ b/icu4c/source/common/uidna.cpp
@@ -205,7 +205,7 @@ _internal_toASCII(const UChar* src, int32_t srcLength,
     UChar b1Stack[MAX_LABEL_BUFFER_SIZE], b2Stack[MAX_LABEL_BUFFER_SIZE];
     //initialize pointers to stack buffers
     UChar  *b1 = b1Stack, *b2 = b2Stack;
-    int32_t b1Len, b2Len, 
+    int32_t b1Len=0, b2Len, 
             b1Capacity = MAX_LABEL_BUFFER_SIZE, 
             b2Capacity = MAX_LABEL_BUFFER_SIZE ,
             reqLength=0;
@@ -224,30 +224,48 @@ _internal_toASCII(const UChar* src, int32_t srcLength,
     UBool useSTD3ASCIIRules = (UBool)((options & UIDNA_USE_STD3_RULES) != 0);
 
     int32_t failPos = -1;
-
-    // step 2    
-    b1Len = usprep_prepare(nameprep, src, srcLength, b1, b1Capacity, namePrepOptions, parseError, status);
     
-    if(*status == U_BUFFER_OVERFLOW_ERROR){
-        // redo processing of string
-        // we do not have enough room so grow the buffer
-        b1 = (UChar*) uprv_malloc(b1Len * U_SIZEOF_UCHAR);
-        if(b1==NULL){
-            *status = U_MEMORY_ALLOCATION_ERROR;
-            goto CLEANUP;
-        }
+    if(srcLength == -1){
+        srcLength = u_strlen(src);
+    }
 
-        *status = U_ZERO_ERROR; // reset error
+    // step 1 
+    for( j=0;j<srcLength;j++){
+        if(src[j] > 0x7F){
+            srcIsASCII = FALSE;
+        }
+        b1[b1Len++] = src[j];
+    }
+    
+    // step 2 is performed only if the source contains non ASCII
+    if(srcIsASCII == FALSE){
         
-        b1Len = usprep_prepare(nameprep, src, srcLength, b1, b1Len, namePrepOptions, parseError, status);
+        // step 2    
+        b1Len = usprep_prepare(nameprep, src, srcLength, b1, b1Capacity, namePrepOptions, parseError, status);
+
+        if(*status == U_BUFFER_OVERFLOW_ERROR){
+            // redo processing of string
+            // we do not have enough room so grow the buffer
+            b1 = (UChar*) uprv_malloc(b1Len * U_SIZEOF_UCHAR);
+            if(b1==NULL){
+                *status = U_MEMORY_ALLOCATION_ERROR;
+                goto CLEANUP;
+            }
+
+            *status = U_ZERO_ERROR; // reset error
+            
+            b1Len = usprep_prepare(nameprep, src, srcLength, b1, b1Len, namePrepOptions, parseError, status);
+        }
     }
     // error bail out
     if(U_FAILURE(*status)){
         goto CLEANUP;
     }
 
-    // step 3 & 4
+    // for step 3 & 4
+    srcIsASCII = TRUE;
     for( j=0;j<b1Len;j++){
+        // check if output of usprep_prepare is all ASCII 
         if(b1[j] > 0x7F){
             srcIsASCII = FALSE;
         }else if(isLDHChar(b1[j])==FALSE){  // if the char is in ASCII range verify that it is an LDH character
@@ -255,7 +273,6 @@ _internal_toASCII(const UChar* src, int32_t srcLength,
             failPos = j;
         }
     }
-    
     if(useSTD3ASCIIRules == TRUE){
         // verify 3a and 3b
         // 3(a) Verify the absence of non-LDH ASCII code points; that is, the
@@ -282,6 +299,7 @@ _internal_toASCII(const UChar* src, int32_t srcLength,
             goto CLEANUP;
         }
     }
+    // Step 4: if the source is ASCII then proceed to step 8
     if(srcIsASCII){
         if(b1Len <= destCapacity){
             uprv_memmove(dest, b1, b1Len * U_SIZEOF_UCHAR);
@@ -341,7 +359,7 @@ _internal_toASCII(const UChar* src, int32_t srcLength,
             goto CLEANUP;
         }
     }
-
+    // step 8: verify the length of lable
     if(reqLength > MAX_LABEL_LENGTH){
         *status = U_IDNA_LABEL_TOO_LONG_ERROR;
     }
diff --git a/icu4c/source/common/utypes.c b/icu4c/source/common/utypes.c
index 02f5f006120..e441a984487 100644
--- a/icu4c/source/common/utypes.c
+++ b/icu4c/source/common/utypes.c
@@ -159,9 +159,9 @@ _uRegexErrorName[U_REGEX_ERROR_LIMIT - U_REGEX_ERROR_START] = {
 static const char * const
 _uIDNAErrorName[U_IDNA_ERROR_LIMIT - U_IDNA_ERROR_START] = {
       "U_IDNA_ERROR_START",
-      "U_IDNA_PROHIBITED_ERROR",
-      "U_IDNA_UNASSIGNED_ERROR",
-      "U_IDNA_CHECK_BIDI_ERROR",
+      "U_STRINGPREP_PROHIBITED_ERROR",
+      "U_STRINGPREP_UNASSIGNED_ERROR",
+      "U_STRINGPREP_CHECK_BIDI_ERROR",
       "U_IDNA_STD3_ASCII_RULES_ERROR",
       "U_IDNA_ACE_PREFIX_ERROR",
       "U_IDNA_VERIFICATION_ERROR",