diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,32 +1,51 @@
 # Changelog
 All notable changes to the `align-equal` project will be documented in this file.
 
+## [1.0.0] - 2025-03-31
+
+### Changed:
+- **`prefixLength`**: Changed return type from `Int` to `Maybe Int`. Now returns `Nothing` if no '=' is found, and `Just` the length of the prefix before the first '=' if it exists. This is a breaking change.
+- Updated documentation for **`prefixLength`** and **`adjustText`** to reflect the new behavior.
+- Bumped package version to `1.0.0` to reflect a breaking change in the API.
+
+### Fixed:
+- Properly handles lines without '=' in **`adjustText`** and **`prefixLength`**.
+
 ## [0.1.1.1] - 2025-03-31
-### Added  
-- Expanded `README.md` with a detailed description of the tool.  
-- Included Vim usage instructions with examples.  
 
-### Miscellaneous  
-- Bumped version to `0.1.1.1` in `align-equal.cabal`.  
+### Added:
+- Expanded `README.md` with a detailed description of the tool.
+- Included Vim usage instructions with examples.
 
+### Miscellaneous:
+- Bumped version to `0.1.1.1` in `align-equal.cabal`.
+
 ## [0.1.1.0] - 2025-03-27
-- **Added**: Extracted core functions into a separate library module (`Data.Text.AlignEqual`).
+
+### Added:
+- Extracted core functions into a separate library module (`Data.Text.AlignEqual`).
   - Refactored original `Main.hs` into executable and library components.
   - Added `src/Data/Text/AlignEqual.hs` with core text alignment logic.
   - Updated `app/Main.hs` to import and use `Data.Text.AlignEqual`.
   - Adjusted `.cabal` file to include a new library section and dependencies.
-- **Added**: Haddock documentation for `Data.Text.AlignEqual`.
+- Added Haddock documentation for `Data.Text.AlignEqual`.
   - Documented module purpose, `prefixLength`, `adjustLine`, and `adjustText` with descriptions, examples, and parameter annotations.
-- **Added**: Included `CHANGELOG.md` in the package documentation via `extra-doc-files` in `.cabal`.
-- **Maintained**: Compatibility with the existing executable.
+- Included `CHANGELOG.md` in the package documentation via `extra-doc-files` in `.cabal`.
+- Maintained compatibility with the existing executable.
 
 ## [0.1.0.1] - 2025-03-10
-- **Fixed**: Relaxed `base` dependency constraint to `>=4.17 && <4.19`.
-- **Fixed**: Removed duplicate `extra-doc-files` entry in `.cabal` file.
-- **Added**: `README.md` with credit to Gabriella Gonzalez.
-- **Added**: Source repository link to `hub.darcs.net` in `.cabal` file.
 
+### Fixed:
+- Relaxed `base` dependency constraint to `>=4.17 && <4.19`.
+- Removed duplicate `extra-doc-files` entry in `.cabal` file.
+
+### Added:
+- `README.md` with credit to Gabriella Gonzalez.
+- Source repository link to `hub.darcs.net` in `.cabal` file.
+
 ## [0.1.0.0] - 2025-03-10
-- **Initial Release**: Basic functionality for aligning text with equal signs.
+
+### Initial Release:
+- Basic functionality for aligning text with equal signs.
   - Implemented as a standalone executable in `Main.hs`.
   - Initial project setup with `.cabal` and `LICENSE` files.
diff --git a/align-equal.cabal b/align-equal.cabal
--- a/align-equal.cabal
+++ b/align-equal.cabal
@@ -1,6 +1,6 @@
 cabal-version: 3.0
 name: align-equal
-version: 0.1.1.1
+version: 1.0.0.0
 license: MIT
 license-file: LICENSE
 author: Joonkyu Park (based on original work by Gabriella Gonzalez)
diff --git a/src/Data/Text/AlignEqual.hs b/src/Data/Text/AlignEqual.hs
--- a/src/Data/Text/AlignEqual.hs
+++ b/src/Data/Text/AlignEqual.hs
@@ -3,28 +3,33 @@
 -- | A module providing functions for text alignment and padding.
 module Data.Text.AlignEqual where
 
-import           Data.Text (Text)
-import qualified Data.Text as T
-import           Safe
+import           Data.Maybe (catMaybes)
+import           Data.Text  (Text)
+import qualified Data.Text  as T
+import           Safe       (maximumMay)
 
 -- | Calculates the number of characters preceding the first '=' sign in a text line.
+-- If no '=' is found, returns `Nothing`. If '=' is found, returns `Just` the length of the prefix before it.
 --
 -- >>> prefixLength "key=value"
--- 3
+-- Just 3
 -- >>> prefixLength "a=b"
--- 1
+-- Just 1
 -- >>> prefixLength "noequals"
--- 8
+-- Nothing
 prefixLength
   :: Text
   -- ^ The input text line
-  -> Int
-  -- ^ The number of characters before the first '=' sign
-prefixLength line = T.length prefix
+  -> Maybe Int
+  -- ^ The number of characters before the first '=' sign, or `Nothing` if no '=' is found, or `Just` the length otherwise
+prefixLength line =
+  if T.null suffix
+    then Nothing
+    else Just (T.length prefix)
   where
-    (prefix, _) = T.breakOn "=" line
+    (prefix, suffix) = T.breakOn "=" line
 
--- | Adjusts the prefix of a text line to a desired length by adding padding.
+-- | Adjusts the prefix of a text line to a desired length by adding padding spaces.
 --
 -- >>> adjustLine 5 "key=value"
 -- "key  =value"
@@ -50,7 +55,8 @@
     newLine = T.concat [ prefix, spaces, suffix ]
 
 -- | Processes multi-line text to align all '=' signs across lines.
---   Adjusts the prefix length of each line to match the maximum prefix length.
+-- It adjusts the prefix length of each line to match the maximum prefix length.
+-- If a line does not contain '=', it will be left as-is.
 --
 -- >>> adjustText "key=value\na=b"
 -- "key=value\na  =b"
@@ -67,8 +73,10 @@
 
     prefixLengths = map prefixLength oldLines
 
+    prefixLengths' = catMaybes prefixLengths
+
     newLines =
-      case maximumMay prefixLengths of
+      case maximumMay prefixLengths' of
         Nothing ->
           []
         Just desiredPrefixLength ->