diff --git a/src/Type/Php/ImplodeFunctionReturnTypeExtension.php b/src/Type/Php/ImplodeFunctionReturnTypeExtension.php
index 4f38ed5137..a23d8443e1 100644
--- a/src/Type/Php/ImplodeFunctionReturnTypeExtension.php
+++ b/src/Type/Php/ImplodeFunctionReturnTypeExtension.php
@@ -62,10 +62,11 @@ public function getTypeFromFunctionCall(
 	private function implode(Type $arrayType, Type $separatorType): Type
 	{
 		if (count($arrayType->getConstantArrays()) > 0 && count($separatorType->getConstantStrings()) > 0) {
+			$isNonEmpty = $arrayType->isIterableAtLeastOnce()->yes();
 			$result = [];
 			foreach ($separatorType->getConstantStrings() as $separator) {
 				foreach ($arrayType->getConstantArrays() as $constantArray) {
-					$constantType = $this->inferConstantType($constantArray, $separator);
+					$constantType = $this->inferConstantType($constantArray, $separator, $isNonEmpty);
 					if ($constantType !== null) {
 						$result[] = $constantType;
 						continue;
@@ -110,7 +111,7 @@ private function implode(Type $arrayType, Type $separatorType): Type
 		return new StringType();
 	}
 
-	private function inferConstantType(ConstantArrayType $arrayType, ConstantStringType $separatorType): ?Type
+	private function inferConstantType(ConstantArrayType $arrayType, ConstantStringType $separatorType, bool $isNonEmpty): ?Type
 	{
 		$sep = $separatorType->getValue();
 		$valueTypes = $arrayType->getValueTypes();
@@ -150,9 +151,16 @@ private function inferConstantType(ConstantArrayType $arrayType, ConstantStringT
 
 		$strings = [];
 		foreach ($partials as $partial) {
+			if ($partial === [] && $isNonEmpty) {
+				continue;
+			}
 			$strings[] = new ConstantStringType(implode($sep, $partial));
 		}
 
+		if ($strings === []) {
+			return null;
+		}
+
 		return TypeCombinator::union(...$strings);
 	}
 
diff --git a/tests/PHPStan/Analyser/nsrt/bug-14558.php b/tests/PHPStan/Analyser/nsrt/bug-14558.php
new file mode 100644
index 0000000000..6f6259d411
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-14558.php
@@ -0,0 +1,81 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Bug14558;
+
+use function PHPStan\Testing\assertType;
+
+/** @return non-negative-int */
+function num_types(string $g): int {  return 0; }
+
+/** @return non-empty-list<non-empty-string> */
+function get_sort_keys(mixed ...$args): array { return ['a']; }
+
+function cond(int $i): bool {  return true; }
+
+
+// Playground 1: with outer foreach loop
+function test1(): void
+{
+	$cols_cat = [ ];
+
+	foreach ([ 'PrV', 'PrA', 'Acc' ] as $g) {
+		$num_types = num_types($g);
+		for ($i = 1; $i <= $num_types; $i++) {
+
+			if (cond($i)) {
+
+				$k = 0;
+				$tmp_sort_keys = [ ];
+				foreach (get_sort_keys($g, $i) as $ce_tri) {
+					$k++;
+					$tmp_sort_alias = "Tri{$k}_Cat_{$g}{$i}";
+					$tmp_sort_keys[$tmp_sort_alias] = $tmp_sort_alias;
+				}
+				assertType('non-falsy-string', implode(',', $tmp_sort_keys));
+				$cols_cat[] = [
+					'g' => $g
+					, 't' => $i
+					, 's' => implode(',', $tmp_sort_keys)
+				];
+
+			}
+		}
+
+	}
+
+	assertType('list<array{g: \'Acc\'|\'PrA\'|\'PrV\', t: int<1, max>, s: non-falsy-string}>', $cols_cat);
+}
+
+// Playground 2: without outer foreach loop
+function test2(): void
+{
+	$cols_cat = [ ];
+
+	$g = 'PrV';
+	$num_types = num_types($g);
+	for ($i = 1; $i <= $num_types; $i++) {
+
+		if (cond($i)) {
+
+			$k = 0;
+			$tmp_sort_keys = [ ];
+			foreach (get_sort_keys($g, $i) as $ce_tri) {
+				$k++;
+				$tmp_sort_alias = "Tri{$k}_Cat_{$g}{$i}";
+				$tmp_sort_keys[$tmp_sort_alias] = $tmp_sort_alias;
+			}
+
+			assertType('non-falsy-string', implode(',', $tmp_sort_keys));
+			$cols_cat[] = [
+				'g' => $g
+				, 't' => $i
+				, 's' => implode(',', $tmp_sort_keys)
+			];
+
+		}
+	}
+
+	assertType('list<array{g: \'PrV\', t: int<1, max>, s: non-falsy-string}>', $cols_cat);
+}