diff --git a/src/generator.js b/src/generator.js
index 7ec005b..f2c6f80 100644
--- a/src/generator.js
+++ b/src/generator.js
@@ -4,6 +4,8 @@
  * @typedef {import('./parse-data')} ParseData
  */
 
+const sprintf = require('sprintf-js').sprintf;
+
 class Generator {
   constructor() {
     /**
@@ -93,7 +95,7 @@ class Generator {
    */
   _getBranch(name) {
     if (this._branches.has(name) === false) {
-      throw new Error('Branch not created: ' + name);
+      throw new Error(sprintf('A branch named \'%s\' not created.', name));
     }
 
     return this._branches.get(name);
diff --git a/src/grammar-support/branch-list.js b/src/grammar-support/branch-list.js
index 5fb1c67..f92303e 100644
--- a/src/grammar-support/branch-list.js
+++ b/src/grammar-support/branch-list.js
@@ -1,3 +1,4 @@
+const sprintf = require('sprintf-js').sprintf;
 const Branch = require('./branch');
 
 /**
@@ -18,7 +19,7 @@ class BranchList {
    */
   add(branch) {
     if (this._branches.has(branch)) {
-      throw new Error('Branch already exists: ' + branch);
+      throw new Error(sprintf('A branch named \'%s\' already exists.', branch));
     }
 
     this._branches.set(branch, new Branch(branch));
@@ -31,7 +32,7 @@ class BranchList {
    */
   get(branch) {
     if (!this._branches.has(branch)) {
-      throw new Error('Branch not created: ' + branch);
+      throw new Error(sprintf('A branch named \'%s\' not created.', branch));
     }
 
     return this._branches.get(branch);
diff --git a/src/grammar-support/tag-list.js b/src/grammar-support/tag-list.js
index 635e949..ef0d557 100644
--- a/src/grammar-support/tag-list.js
+++ b/src/grammar-support/tag-list.js
@@ -1,3 +1,5 @@
+const sprintf = require('sprintf-js').sprintf;
+
 class TagList {
   constructor() {
     /**
@@ -13,7 +15,7 @@ class TagList {
    */
   add(tag) {
     if (this._tags.has(tag)) {
-      throw new Error('Tag already exists: ' + tag);
+      throw new Error(sprintf('Tag \'%s\' already exists.', tag));
     }
 
     this._tags.add(tag);
diff --git a/test/input_error_semantics.js b/test/input_error_semantics.js
index 9d5ddfa..7e3e755 100644
--- a/test/input_error_semantics.js
+++ b/test/input_error_semantics.js
@@ -73,7 +73,7 @@ module.exports = [
       git branch foo
     `,
     line: 6,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
   {
     case: 'create branch - already created - 02',
@@ -85,7 +85,7 @@ module.exports = [
       git checkout -b foo
     `,
     line: 6,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
   {
     case: 'create branch - already created - 03',
@@ -97,7 +97,7 @@ module.exports = [
       git switch -c foo
     `,
     line: 6,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
   {
     case: 'create branch - already created - 04',
@@ -109,7 +109,7 @@ module.exports = [
       git branch foo
     `,
     line: 6,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
   {
     case: 'create branch - already created - 05',
@@ -121,7 +121,7 @@ module.exports = [
       git checkout -b foo
     `,
     line: 6,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
   {
     case: 'create branch - already created - 06',
@@ -133,7 +133,7 @@ module.exports = [
       git switch -c foo
     `,
     line: 6,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
   {
     case: 'create branch - already created - 07',
@@ -145,7 +145,7 @@ module.exports = [
       git branch foo
     `,
     line: 6,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
   {
     case: 'create branch - already created - 08',
@@ -157,7 +157,7 @@ module.exports = [
       git checkout -b foo
     `,
     line: 6,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
   {
     case: 'create branch - already created - 09',
@@ -169,7 +169,7 @@ module.exports = [
       git switch -c foo
     `,
     line: 6,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
   {
     case: 'create branch - already created - 10',
@@ -180,7 +180,7 @@ module.exports = [
       git branch foo
     `,
     line: 5,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
   {
     case: 'create branch - already created - 11',
@@ -191,7 +191,7 @@ module.exports = [
       git checkout -b foo
     `,
     line: 5,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
   {
     case: 'create branch - already created - 12',
@@ -202,7 +202,7 @@ module.exports = [
       git switch -c foo
     `,
     line: 5,
-    message: 'Branch already exists: foo',
+    message: "A branch named 'foo' already exists.",
   },
 
   // merge - branch not created
@@ -213,7 +213,7 @@ module.exports = [
       git merge foo
     `,
     line: 3,
-    message: 'Branch not created: foo',
+    message: "A branch named 'foo' not created.",
   },
   {
     case: 'merge - branch not created - 02',
@@ -223,7 +223,7 @@ module.exports = [
       git merge foo
     `,
     line: 4,
-    message: 'Branch not created: foo',
+    message: "A branch named 'foo' not created.",
   },
 
   // merge - no commits on branch
@@ -287,7 +287,7 @@ module.exports = [
       git tag v1
     `,
     line: 5,
-    message: 'Tag already exists: v1',
+    message: "Tag 'v1' already exists.",
   },
   {
     case: 'tag - already exists - 02',
@@ -299,7 +299,7 @@ module.exports = [
       git tag v1
     `,
     line: 6,
-    message: 'Tag already exists: v1',
+    message: "Tag 'v1' already exists.",
   },
 
   // tag - no commits on branch