diff --git a/src/DataFrame.php b/src/DataFrame.php
index 4840c00..414bcbf 100644
--- a/src/DataFrame.php
+++ b/src/DataFrame.php
@@ -38,7 +38,7 @@ final class DataFrame extends DataFrameCore
      * @return DataFrame
      * @since  0.1.0
      */
-    public static function fromCSV($fileName, $options = []): self
+    public static function fromCSV(string $fileName, array $options = []): self
     {
         $csv = new CSV($fileName);
         $data = $csv->loadFile($options);
@@ -54,7 +54,7 @@ public static function fromCSV($fileName, $options = []): self
      * @throws \CondorcetPHP\Oliphant\Exceptions\FileExistsException
      * @since  0.1.0
      */
-    public function toCSV($fileName, $options = []): self
+    public function toCSV(string $fileName, array $options = []): self
     {
         $csv = new CSV($fileName);
         $csv->saveFile($this->data, $options);
@@ -70,7 +70,7 @@ public function toCSV($fileName, $options = []): self
      * @return DataFrame
      * @since  0.1.0
      */
-    public static function fromFWF($fileName, array $colSpecs, array $options = []): self
+    public static function fromFWF(string $fileName, array $colSpecs, array $options = []): self
     {
         $fwf = new FWF($fileName);
         $data = $fwf->loadFile($colSpecs, $options);
@@ -98,7 +98,7 @@ public static function fromXLSX($fileName, array $options = []): self
      * @param $worksheetTitle
      * @since  0.3.0
      */
-    public function toXLSXWorksheet(Spreadsheet &$excel, $worksheetTitle): Worksheet
+    public function toXLSXWorksheet(Spreadsheet &$excel, string $worksheetTitle): Worksheet
     {
         return XLSX::saveToWorksheet($excel, $worksheetTitle, $this->data, $this->columns);
     }
@@ -110,7 +110,7 @@ public function toXLSXWorksheet(Spreadsheet &$excel, $worksheetTitle): Worksheet
      * @return DataFrame
      * @since  0.3.0
      */
-    public static function fromSQL($sqlQuery, PDO $pdo): self
+    public static function fromSQL(string $sqlQuery, PDO $pdo): self
     {
         $sql = new SQL($pdo);
         $data = $sql->select($sqlQuery);
diff --git a/src/DataFrameCore.php b/src/DataFrameCore.php
index 58e9815..ad39205 100644
--- a/src/DataFrameCore.php
+++ b/src/DataFrameCore.php
@@ -120,7 +120,7 @@ public function apply(Closure $f): self
      * @return DataFrameCore
      * @since  0.1.0
      */
-    public function applyIndexMap(array $map, mixed $column = null)
+    public function applyIndexMap(array $map, ?string $column = null)
     {
         return $this->apply(static function (&$row, $i) use ($map, $column) {
             if (isset($map[$i])) {
@@ -214,7 +214,7 @@ public function query(string $sql, ?PDO $pdo = null): self
      * @throws InvalidColumnException
      * @since  0.1.0
      */
-    public function mustHaveColumn(mixed $columnName): void
+    public function mustHaveColumn(string $columnName): void
     {
         if ($this->hasColumn($columnName) === false) {
             throw new InvalidColumnException("{$columnName} doesn't exist in DataFrame");
@@ -228,7 +228,7 @@ public function mustHaveColumn(mixed $columnName): void
      * @return bool
      * @since  0.1.0
      */
-    public function hasColumn(mixed $columnName): bool
+    public function hasColumn(string $columnName): bool
     {
         if (array_search($columnName, $this->columns, true) === false) {
             return false;
@@ -244,7 +244,7 @@ public function hasColumn(mixed $columnName): bool
      * @param $columnName
      * @since 0.1.0
      */
-    private function addColumn($columnName): void
+    private function addColumn(string $columnName): void
     {
         if (!$this->hasColumn($columnName)) {
             $this->columns[] = $columnName;
@@ -274,7 +274,7 @@ private function addColumns(array $columnNames): void
      * @param $from
      * @param $to
      */
-    public function renameColumn($from, $to): void
+    public function renameColumn(string $from, string $to): void
     {
         $this->mustHaveColumn($from);
 
@@ -298,7 +298,7 @@ public function renameColumn($from, $to): void
      * @param $columnName
      * @since 0.1.0
      */
-    public function removeColumn($columnName): void
+    public function removeColumn(string $columnName): void
     {
         unset($this[$columnName]);
     }
@@ -360,7 +360,7 @@ public function preg_replace($pattern, $replacement): self
      * @param null|string $toDateFormat The date format of the output.
      * @throws Exception
      */
-    public function convertTypes(array $typeMap, $fromDateFormat = null, $toDateFormat = null): void
+    public function convertTypes(array $typeMap, array|string|null $fromDateFormat = null, ?string $toDateFormat = null): void
     {
         foreach ($this as $i => $row) {
             foreach ($typeMap as $column => $type) {
@@ -379,7 +379,7 @@ public function convertTypes(array $typeMap, $fromDateFormat = null, $toDateForm
         }
     }
 
-    private function convertNumeric($value): int|float
+    private function convertNumeric(mixed $value): int|float
     {
         if (is_numeric($value)) {
             return $value;
@@ -396,7 +396,7 @@ private function convertNumeric($value): int|float
         return (\is_int($value / 1)) ? \intval($value) : $value;
     }
 
-    private function convertInt($value): int
+    private function convertInt(mixed $value): int
     {
         if (empty($value)) {
             return 0;
@@ -413,7 +413,7 @@ private function convertInt($value): int
         return \intval(str_replace(',', '', $value));
     }
 
-    private function convertDatetime($value, $fromFormat, $toFormat): string
+    private function convertDatetime(mixed $value, array|string|null $fromFormat, string $toFormat): string
     {
         if (empty($value)) {
             return DateTime::createFromFormat('Y-m-d', '0001-01-01')->format($toFormat);
@@ -529,7 +529,7 @@ public function sortValues(array|string $by, bool $ascending = true): void
             $by = [$by];
         }
 
-        usort($this->data, static function ($row_a, $row_b) use ($by, $ascending): int {
+        usort($this->data, static function (array $row_a, array $row_b) use ($by, $ascending): int {
             foreach ($by as $col) {
                 if ($row_a[$col] > $row_b[$col]) {
                     return $ascending ? 1 : -1;
@@ -609,7 +609,7 @@ public function offsetGet(mixed $columnName): mixed
      * @throws DataFrameException
      * @since  0.1.0
      */
-    public function offsetSet($targetColumn, $rightHandSide): void
+    public function offsetSet(mixed $targetColumn, mixed $rightHandSide): void
     {
         if ($rightHandSide instanceof DataFrame) {
             $this->offsetSetDataFrame($targetColumn, $rightHandSide);
@@ -631,7 +631,7 @@ public function offsetSet($targetColumn, $rightHandSide): void
      * @throws DataFrameException
      * @since  0.1.0
      */
-    private function offsetSetDataFrame($targetColumn, DataFrame $df): void
+    private function offsetSetDataFrame(string $targetColumn, DataFrame $df): void
     {
         if (\count($df->columns()) !== 1) {
             $msg = 'Can only set a new column from a DataFrame with a single ';
@@ -664,7 +664,7 @@ private function offsetSetDataFrame($targetColumn, DataFrame $df): void
      * @param Closure $f
      * @since 0.1.0
      */
-    private function offsetSetClosure($targetColumn, Closure $f): void
+    private function offsetSetClosure(string $targetColumn, Closure $f): void
     {
         foreach ($this as $i => $row) {
             $this->data[$i][$targetColumn] = $f($row[$targetColumn]);
@@ -691,7 +691,6 @@ private function offsetSetValue(?string $targetColumn, mixed $value): void
                 $this->data[$i][$targetColumn] = $value;
             }
         } elseif (\is_array($value)) {
-            $this->addColumns(array_keys($value));
             $this->data[] = $value;
         }
     }
diff --git a/src/IO/XLSX.php b/src/IO/XLSX.php
index 6a2a20c..154946a 100644
--- a/src/IO/XLSX.php
+++ b/src/IO/XLSX.php
@@ -96,7 +96,7 @@ public function loadFile(array $options): array
      * @throws \PhpOffice\PhpSpreadsheet\Exception
      * @since  0.3.0
      */
-    public static function saveToWorksheet(Spreadsheet &$excel, $worksheetTitle, array $data, array $columns): Worksheet
+    public static function saveToWorksheet(Spreadsheet &$excel, string $worksheetTitle, array $data, array $columns): Worksheet
     {
         // Check if this is a brand new spreadsheet
         if ($excel->getSheetCount() === 1) {