diff --git a/api/tests/integration/ref/formats/idt_to_ket.py.out b/api/tests/integration/ref/formats/idt_to_ket.py.out
index 2e41b85d21..91da009843 100644
--- a/api/tests/integration/ref/formats/idt_to_ket.py.out
+++ b/api/tests/integration/ref/formats/idt_to_ket.py.out
@@ -31,6 +31,7 @@ idt_unresolved_many.ket:SUCCEED
 idt_unsplit.ket:SUCCEED
 Test '!+A-$#12w12r23e32e33': got expected error 'Invalid symbols in the sequence: !,-,$,#,1,2,w,1,2,2,3,e,3,2,e,3,3'
 Test '(Y:)': got expected error 'Invalid IDT ambiguous monomer (Y:)'
+Test '(Y:000010af)': got expected error 'Invalid number 'af''
 Test '(YY:00330067)': got expected error 'Invalid mixed base - only numerical index allowed.'
 Test '+/5Phos/A': got expected error 'Sugar prefix could not be used with modified monomer.'
 Test '/': got expected error 'Unexpected end of data'
diff --git a/api/tests/integration/tests/formats/idt_to_ket.py b/api/tests/integration/tests/formats/idt_to_ket.py
index 3b4e85798d..1fa58525f3 100644
--- a/api/tests/integration/tests/formats/idt_to_ket.py
+++ b/api/tests/integration/tests/formats/idt_to_ket.py
@@ -103,6 +103,7 @@ def find_diff(a, b):
     "r(B1:50003000)(B1)": "Unknown mixed base 'B1'",
     "(YY:00330067)": "Invalid mixed base - only numerical index allowed.",
     "(Y:)": "Invalid IDT ambiguous monomer (Y:)",
+    "(Y:000010af)": "Invalid number 'af'",
 }
 for idt_seq in sorted(idt_errors.keys()):
     error = idt_errors[idt_seq]
diff --git a/core/indigo-core/molecule/src/sequence_loader.cpp b/core/indigo-core/molecule/src/sequence_loader.cpp
index a4577433eb..c37279a4a2 100644
--- a/core/indigo-core/molecule/src/sequence_loader.cpp
+++ b/core/indigo-core/molecule/src/sequence_loader.cpp
@@ -1096,8 +1096,18 @@ void SequenceLoader::loadIdt(KetDocument& document)
                             check_mixed_base(mixed_base);
                             if (ratios_str.size() != 8)
                                 throw Exception("Invalid IDT ambiguous monomer %s", idt_alias.c_str());
-                            ratios.emplace(std::array<float, 4>{std::stof(ratios_str.substr(0, 2)), std::stof(ratios_str.substr(2, 2)),
-                                                                std::stof(ratios_str.substr(4, 2)), std::stof(ratios_str.substr(6, 2))});
+                            auto stof = [](const std::string& arg) -> float {
+                                try
+                                {
+                                    return std::stof(arg);
+                                }
+                                catch (...)
+                                {
+                                    throw Error("Invalid number '%s'", arg.c_str());
+                                }
+                            };
+                            ratios.emplace(std::array<float, 4>{stof(ratios_str.substr(0, 2)), stof(ratios_str.substr(2, 2)), stof(ratios_str.substr(4, 2)),
+                                                                stof(ratios_str.substr(6, 2))});
                             idt_alias = '(' + mixed_base + ')';
                             mixed_base = mixed_base[0];
                         }
@@ -1491,7 +1501,14 @@ SequenceLoader::MonomerInfo SequenceLoader::readHelmMonomer(KetDocument& documen
             auto& opt = options.second.emplace_back(opt_alias, std::optional<float>());
             if (count.size() > 0)
             {
-                opt.second = std::stof(count);
+                try
+                {
+                    opt.second = std::stof(count);
+                }
+                catch (...)
+                {
+                    throw Error("Invalid number '%s'", count.c_str());
+                }
                 no_counts = false;
             }
             if (ch == ')')