diff --git a/api/tests/integration/ref/formats/helm_to_ket.py.out b/api/tests/integration/ref/formats/helm_to_ket.py.out
index 5a20ee903e..93f3e1ade4 100644
--- a/api/tests/integration/ref/formats/helm_to_ket.py.out
+++ b/api/tests/integration/ref/formats/helm_to_ket.py.out
@@ -15,6 +15,7 @@ Test 'CHEM1{[A6OH]}|PEPTIDE1{A}$CHEM1,PEPTIDE1,1:R4-1:R1$$$V2.0': got expected e
 Test 'CHEM1{[A6OH]}|PEPTIDE1{A}$CHEM10,PEPTIDE1,1:R2-1:R1$$$V2.0': got expected error 'Polymer 'CHEM10' not found.'
 Test 'CHEM1{[MCC]}|RNA1{R(A)P.R(C)P.R(G)P.R(T)P.R(U)P}$RNA1,PEPTIDE1,15:R2-1:R1$$$V2.0': got expected error 'Polymer 'PEPTIDE1' not found.'
 Test 'CHEM1{[MCC]}|RNA1{R(U)P}$CHEM1,RNA1,1:R1-1:R2$$$V2.0': got expected error 'Monomer 'R' attachment point 'R2' already connected to monomer'monomer3' attachment point 'R1''
+Test 'PEPTIDE1{(A:+C:0.1)}$$$$V2.0': got expected error 'Unexpected symbol. Expected digit but found '+''
 Test 'PEPTIDE1{(A:1.5+C:aaaa)}$$$$V2.0': got expected error 'Unexpected symbol. Expected '+' or ',' but found '.''
 Test 'PEPTIDE1{A'2'}$$$$V2.0': got expected error 'Repeating not supported now.'
 Test 'PEPTIDE1{D-gGlu}$$$$V2.0': got expected error 'Unexpected symbol. Expected '.' or '}' but found '-'.'
diff --git a/api/tests/integration/tests/formats/helm_to_ket.py b/api/tests/integration/tests/formats/helm_to_ket.py
index 44d23a081b..0f92f41076 100644
--- a/api/tests/integration/tests/formats/helm_to_ket.py
+++ b/api/tests/integration/tests/formats/helm_to_ket.py
@@ -70,6 +70,7 @@ def find_diff(a, b):
     "PEPTIDE1{(A:1.5+C:aaaa)}$$$$V2.0": "Unexpected symbol. Expected '+' or ',' but found '.'",
     "RNA1{R(bla-bla-bla)p}$$$$V2.0": "Unexpected symbol. Expected ')' but found 'l'.",
     "PEPTIDE1{D-gGlu}$$$$V2.0": "Unexpected symbol. Expected '.' or '}' but found '-'.",
+    "PEPTIDE1{(A:+C:0.1)}$$$$V2.0": "Unexpected symbol. Expected digit but found '+'",
 }
 for helm_seq in sorted(helm_errors.keys()):
     error = helm_errors[helm_seq]
diff --git a/core/indigo-core/molecule/src/sequence_loader.cpp b/core/indigo-core/molecule/src/sequence_loader.cpp
index e19f05f3e0..db173db302 100644
--- a/core/indigo-core/molecule/src/sequence_loader.cpp
+++ b/core/indigo-core/molecule/src/sequence_loader.cpp
@@ -1424,7 +1424,7 @@ int SequenceLoader::readCount(std::string& count, Scanner& _scanner)
             ch = _scanner.lookNext();
         }
         if (count.size() == 0)
-            throw Error("Invalid number.");
+            throw Error("Unexpected symbol. Expected digit but found '%c'", ch);
     }
     return ch;
 }