diff --git a/opennlp-core/opennlp-runtime/src/main/java/opennlp/tools/sentdetect/SentenceDetectorME.java b/opennlp-core/opennlp-runtime/src/main/java/opennlp/tools/sentdetect/SentenceDetectorME.java
index ddcc3388c..9b113e120 100644
--- a/opennlp-core/opennlp-runtime/src/main/java/opennlp/tools/sentdetect/SentenceDetectorME.java
+++ b/opennlp-core/opennlp-runtime/src/main/java/opennlp/tools/sentdetect/SentenceDetectorME.java
@@ -345,7 +345,8 @@ protected boolean isAcceptableBreak(CharSequence s, int fromIndex, int candidate
       if (tokenPosition == -1) {
         continue; // skip fast
       }
-      final char prevChar = s.charAt(tokenPosition - 1);
+
+      final char prevChar = s.charAt(tokenPosition == 0 ? tokenPosition : tokenPosition - 1);
       int tokenLength = token.length();
       if (tokenPosition + tokenLength < candidateIndex || tokenPosition > candidateIndex ||
         /*
diff --git a/opennlp-core/opennlp-runtime/src/test/java/opennlp/tools/sentdetect/SentenceDetectorMEGermanTest.java b/opennlp-core/opennlp-runtime/src/test/java/opennlp/tools/sentdetect/SentenceDetectorMEGermanTest.java
index d95a1eeca..5417fe170 100644
--- a/opennlp-core/opennlp-runtime/src/test/java/opennlp/tools/sentdetect/SentenceDetectorMEGermanTest.java
+++ b/opennlp-core/opennlp-runtime/src/test/java/opennlp/tools/sentdetect/SentenceDetectorMEGermanTest.java
@@ -151,6 +151,24 @@ void testSentDetectWithUseTokenEndFalse() {
         () -> assertEquals(2, probs.length));
   }
 
+  /*
+    * A reproducer and test for OPENNLP-1781.
+   */
+  @Test
+  void testSentDetectWithAbbreviationsAtSentenceStart() {
+    prepareResources(true);
+
+    final String sent1 = "S. Träume sind eine Verbindung von Gedanken.";
+
+    final String[] sents = sentenceDetector.sentDetect(sent1);
+    final double[] probs = sentenceDetector.probs();
+
+    assertAll(
+        () -> assertEquals(1, sents.length),
+        () -> assertEquals(sent1, sents[0]),
+        () -> assertEquals(1, probs.length));
+  }
+
   /*
    * A reproducer and test for OPENNLP-1767.
    * It checks that sentence detection with common abbreviations works correctly,
@@ -163,6 +181,7 @@ void testSentDetectWithUseTokenEndFalse() {
       "Der Auto stand schief. Wer hat es dort geparkt?",
       "Es lag am DBMS. Die Performance muss verbessert werden.",
       "Siehe Buch S. 17f. Dort ist es zu finden.",
+      "S. Buch S. 17f. Dort ist es zu finden.", // OPENNLP-1781
       "Sie trank einen Mocca. Er schmeckte ihr!",
       "Der Anker hängt zu Beginn des Bugs. Es ist vertaut.",
       "Das Verfahren testet auf HIV. Es ist präzise."