diff --git a/README.md b/README.md
index a522fee..775e1af 100644
--- a/README.md
+++ b/README.md
@@ -98,12 +98,12 @@ Also, this mark can escape `{` and `}`.
 ##### Theme for argument
 
 A logging function created by this function can take multiple arguments.
-A themed text to be converted to an argument has same format with a normal themed text but its theme name is a number starting from 1, which indicates the index of the argument, like `{2}` or `{2: alternative text }`.
+A themed text to be converted to an argument has same format with a normal themed text but its theme name is a number starting from 1, which indicates the index of the argument, like `{2}` or `{2: Explanatory text }`.
 
 A block content in a theme block for argument is outputted alternatively when there is no argument corresponding to a number of a theme name.
 
 * `{ 1 }` → replaced with the second argument (the first argument except the themed text) of logging function, or an empty string if the second argument is not given.
-* `{ 3 : yyyy }` → replaced with the fourth argument (the third argument except the themed text) of logging function, or `'yyyy'` if the fourth argument is not given.
+* `{ 3 : yyyy }` → replaced with the fourth argument (the third argument except the themed text) of logging function, or an empty string if the fourth argument is not given. (`'yyyy'` is not used.)
 
 #### Parameters:
 
diff --git a/lib/arg-theme.js b/lib/arg-theme.js
index 92602cc..c5da0a4 100644
--- a/lib/arg-theme.js
+++ b/lib/arg-theme.js
@@ -9,7 +9,7 @@ function isArgTheme(themeName) {
 function convertArgTheme(args, themeName, nodeText) {
   var argIndex = toInteger(themeName);
   if (!argIndex || argIndex >= args.length || argIndex <= 0) {
-    return nodeText || '';
+    return '';
   }
 
   return args[argIndex];
diff --git a/test/lib/apply-theme.test.js b/test/lib/apply-theme.test.js
index f0d2285..7a39a2f 100644
--- a/test/lib/apply-theme.test.js
+++ b/test/lib/apply-theme.test.js
@@ -150,7 +150,7 @@ describe('lib/apply-theme', function() {
       'This text has arg-theme: A1, true and 222');
 
     expect(apply(parsed, themeSet)).to.equal(
-      'This text has arg-theme: ,  and Two');
+      'This text has arg-theme: ,  and ');
   });
 
   it('Should support theme defined by nested properties', function() {
diff --git a/test/lib/arg-theme.test.js b/test/lib/arg-theme.test.js
index 92318be..43ad02e 100644
--- a/test/lib/arg-theme.test.js
+++ b/test/lib/arg-theme.test.js
@@ -22,15 +22,15 @@ describe('lib/arg-theme', function() {
 
   it('Should convert an arg identifier to an argument value', function() {
     var args = ['Arg0', 'Arg1', 'Arg2', 'Arg3'];
-    expect(argTheme.convert(args, '-1', 'X1-')).to.equal('X1-');
-    expect(argTheme.convert(args, '0', 'X0')).to.equal('X0');
+    expect(argTheme.convert(args, '-1', 'X1-')).to.equal('');
+    expect(argTheme.convert(args, '0', 'X0')).to.equal('');
     expect(argTheme.convert(args, '1', 'X1')).to.equal('Arg1');
     expect(argTheme.convert(args, '2', 'X2')).to.equal('Arg2');
     expect(argTheme.convert(args, '3', 'X3')).to.equal('Arg3');
-    expect(argTheme.convert(args, '4', 'X4')).to.equal('X4');
+    expect(argTheme.convert(args, '4', 'X4')).to.equal('');
 
-    expect(argTheme.convert(args, '', 'X')).to.equal('X');
-    expect(argTheme.convert(args, 'a', 'X')).to.equal('X');
+    expect(argTheme.convert(args, '', 'X')).to.equal('');
+    expect(argTheme.convert(args, 'a', 'X')).to.equal('');
   });
 
 });
diff --git a/test/theming-log.test.js b/test/theming-log.test.js
index 8477a9f..913196b 100644
--- a/test/theming-log.test.js
+++ b/test/theming-log.test.js
@@ -183,11 +183,11 @@ describe('theming-log', function() {
         expect(logBuf).to.equal('This text has arg-theme: Arg2 and Arg1\n');
       });
 
-      it('Should replace arg-themes to block text when corresponding arg are' +
-      '\n\tnot exist', function() {
+      it('Should replace arg-themes to an empty string when corresponding' +
+      '\n\targ are not exist', function() {
         var log = themingLog(themes, origLogger);
         log('This text has arg-theme: {5} and {3: Three}', 'Arg1', 'Arg2');
-        expect(logBuf).to.equal('This text has arg-theme:  and Three\n');
+        expect(logBuf).to.equal('This text has arg-theme:  and \n');
       });
 
       it('Should replace arg-theme in nested theme', function() {